Fabrication of silicon-on-insulator structure using plasma immersion ion implantation

ABSTRACT

A method of fabricating a silicon-on-insulator structure having a silicon surface layer in a semiconductor workpiece, is carried out by maintaining the workpiece at an elevated temperature and producing an oxygen-containing plasma in the chamber while applying a bias to the workpiece and setting the bias to a level corresponding to an implant depth in the workpiece below the silicon surface layer to which oxygen atoms are to be implanted, whereby to form an oxygen-implanted layer in the workpiece having an oxygen concentration distribution generally centered at the implant depth and having a finite oxygen concentration in the silicon surface layer. The oxygen concentration in the silicon surface layer is then reduced to permit epitaxial silicon deposition.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/646,533, filed Aug. 22, 2003, entitled PLASMA IMMERSION IONIMPLANTATION PROCESS USING A PLASMA SOURCE HAVING LOW DISSOCIATION ANDLOW MINIMUM PLASMA VOLTAGE, by Kenneth Collins, et al., which is acontinuation-in-part of U.S. application Ser. No. 10/164,327, filed Jun.5, 2002, entitled MAGNETICALLY EXCITED TORROIDAL PLASMA SOURCE WITHMAGNETIC CONTROL OF ION DISTRIBUTION, by Kenneth Collins, et al., andassigned to the present assignee.

The following applications contain subject matter related to the presentinvention:

-   -   U.S. patent application Ser. No. 10/646,458, filed Aug. 22,        2003, entitled PLASMA IMMERSION ION IMPLANTATION APPARATUS        INCLUDING A PLASMA SOURCE HAVING LOW DISSOCIATION AND LOW        MINIMUM PLASMA VOLTAGE, by Kenneth Collins, et al.;    -   U.S. patent application Ser. No. 10/646,460, filed Aug. 22,        2003, entitled PLASMA IMMERSION ION IMPLANTATION SYSTEM        INCLUDING AN INDUCTIVELY COUPLED PLASMA SOURCE HAVING LOW        DISSOCIATION AND LOW MINIMUM PLASMA VOLTAGE, by Kenneth Collins        et al.; U.S. patent application Ser. No. 10/646,467, filed Aug.        22, 2003, entitled PLASMA IMMERSION IMPLANTATION PROCESS USING        AN INDUCTIVELY COUPLED PLASMA SOURCE HAVING LOW DISSOCIATION AND        LOW MINIMUM PLASMA VOLTAGE, by Kenneth Collins, et al.; U.S.        patent application Ser. No. 10/646,526, filed Aug. 22, 2003,        entitled PLASMA IMMERSION ION IMPLANTATION SYSTEM INCLUDING A        CAPACITIVELY COUPLED PLASMA SOURCE HAVING LOW DISSOCIATION AND        LOW MINIMUM PLASMA VOLTAGE, by Kenneth Collins, et al.;    -   U.S. patent application Ser. No. 10/646,527, filed Aug. 22,        2003, entitled PLASMA IMMERSION ION IMPLANTATION SYSTEM        INCLUDING A CAPACITIVELY COUPLED PLASMA SOURCE HAVING LOW        DISSOCIATION AND LOW MINIMUM PLASMA VOLTAGE, by Kenneth Collins,        et al.; U.S. patent application Ser. No. 10/646,528, filed Aug.        22, 2003, entitled PLASMA IMMERSION IMPLANTATION APPARATUS        INCLUDING AN INDUCTIVELY COUPLED PLASMA SOURCE HAVING LOW        DISSOCIATION AND LOW MINIMUM PLASMA VOLTAGE, by Kenneth Collins,        et al.; U.S. patent application Ser. No. 10/646,532, filed Aug.        22, 2003, entitled PLASMA IMMERSION ION IMPLANTATION APPARATUS        INCLUDING A CAPACITIVELY COUPLED PLASMA SOURCE HAVING LOW        DISSOCIATION AND LOW MINIMUM PLASMA VOLTAGE, by Kenneth Collins,        et al.;    -   U.S. patent application Ser. No. 10/646,612, filed Aug. 22,        2003, entitled PLASMA IMMERSION ION IMPLANTATION PROCESS USING A        CAPACITIVELY COUPLED PLASMA SOURCE HAVING LOW DISSOCIATION AND        LOW MINIMUM PLASMA VOLTAGE, by Kenneth Collins, et al.

BACKGROUND OF THE INVENTION

Integrated circuit switching speeds are enhanced by forming transistorsin silicon-on-insulator (SOI) structures. An SOI structure consists ofcrystalline silicon islands on an insulator layer of a bulk siliconsubstrate, transistors being formed in or on the individual islands.Device speed is enhanced because the electrical isolation of each islandfrom the bulk substrate and from other islands reduces capacitiveloading of the transistors from the bulk and from devices in the otherislands. The problem is that it is difficult to provide a high qualitycrystalline semiconductor silicon layer over an insulator layer becauseformation of an underlying insulator layer typically introduces defectsinto the overlying semiconductor silicon layer. Such defects may renderit impossible to form working transistors in the overlying semiconductorlayer. One way of avoiding such problems is to use ion beam implantationto implant oxygen atoms well below the bulk silicon surface of a siliconwafer, leaving a surface layer of pure silicon over an underlying layer(or “box”) of silicon containing implanted oxygen atoms. The crystallinestructure of the silicon surface layer may be preserved despite thetransmission of the ion beam therethrough by maintaining the wafertemperature at about 600 degrees C. during the oxygen ion implantationprocess. Thereafter, the wafer is annealed in order to causesubstitution of the implanted oxygen atoms into the crystal structure ofthe “box” to form a silicon dioxide insulator layer. The energy of theoxygen ion beam is selected to place the silicon dioxide box at thedesired depth below the substrate surface, this depth determining thethickness of the crystalline silicon surface layer. The thickness of thesilicon surface layer may be enhanced by epitaxial growth of crystallinesilicon, using chemical vapor deposition, for example. While thisprocess is precise and produces a high quality crystalline siliconsurface layer, it is prohibitively expensive, because the oxygen beamion implantation process requires many hours for a single silicon wafer,which is unacceptable.

One way around this difficulty is a layer transfer technique, in which asilicon dioxide insulator layer is efficiently formed on the surface ofa silicon wafer using a standard furnace oxidation process. Theoverlying pure semiconductor layer is then provided by bonding a thinsilicon wafer to the silicon dioxide surface of the bulk wafer. Whilethis technique is more cost efficient, it produces a lower qualitysemiconductor layer. This is because the thin silicon wafer bonded tothe bulk layer is formed by slicing a standard silicon wafer to producethe thin wafer. The slicing process produces a surface with a relativelyhigh defect density. The slicing may be accomplished by implantinghydrogen (which is a relatively fast process due to the low atomic massof the hydrogen) to form a defect layer at the desired depth. Thehydrogen implanted wafer is then sliced along the defect layer to form athin silicon wafer. The bulk wafer and the thin wafer are bondedback-to-back, leaving the sliced surface of the thin wafer as thesurface in which devices (transistors) are to be formed. Defects in thesliced surface can be reduced by annealing, but some defects willremain, thus compromising device quality in the final product.

In view of the foregoing difficulties of the layer transfer technique,attempts have been made to revert to the ion implantation techniquediscussed above for forming the underlying silicon dioxide insulatorlayer, and overcoming the inefficiency (slowness) of the oxygen ion beamimplantation process by implanting oxygen using plasma immersion ionimplantation. Plasma immersion ion implantation is much faster than ionbeam implantation because it implants ions over the entire wafersurface, in contrast to the narrow beam of an ion beam implant processwhich much be raster scanned over the wafer surface at a relatively slowrate. Moreover, the ion flux of the plasma immersion ion implantationprocess may be increased as desired by increasing the plasma sourcepower. One difficulty with the plasma immersion ion implantation processis that it is incapable of producing sufficient ion energies(implantation depth) to avoid implanting oxygen atoms in the wafersurface layer. The silicon surface layer must be fairly free of defects,such as oxygen atoms, in order to be susceptible of epitaxial growth ofadditional crystalline silicon on the silicon surface layer.Specifically, the oxygen concentration in the silicon surface layershould not exceed a certain threshold, typically about 10¹⁸ cm−3 incertain cases. This is because defects in the silicon surface layer arereplicated in any epitaxial silicon layer that is deposited thereover,and such defects may render the epitaxial silicon layer an amorphous orpolycrystalline non-semiconductor, rather than a crystallinesemiconductor. Typical plasma immersion ion implantation reactors cannotproduce oxygen ion energies sufficient to minimize implanted oxygen inthe silicon surface layer below the threshold for epitaxialsemiconductor silicon growth. Ion energy is increased by increasing theplasma bias power applied to the wafer support pedestal of the plasmaimmersion ion implantation reactor. However, it has been found thatplasma bias power cannot be sufficiently increased to avoid implantingoxygen in the silicon surface layer without causing arcing in thevicinity of the wafer support pedestal, and a consequently loss ofcontrol over the plasma process.

Another problem with the use plasma immersion ion implantation to formSOI structures arises in the preparation of the silicon surface layerfor the epitaxial silicon deposition step. Prior to the step ofepitaxial silicon deposition, the surface must be cleaned to removeimpurities, such as a native oxide, for example. The problem is thatplasma used during the oxygen ion implantation and/or plasma used duringthe surface cleaning step tends to remove material from the reactorchamber interior surfaces, creating contamination (e.g., aluminum) thatis deposited on the silicon surface prior to silicon epitaxialdeposition. Such contamination creates defects, which can render theepitaxially deposited silicon unsuitable.

What is needed is a way of performing plasma immersion oxygen ionimplantation so as to leave an SOI silicon surface layer that isrelatively free of oxygen atoms without increasing plasma bias power.What is also needed is a plasma reactor that does not contaminate thesilicon layer through plasma interaction with chamber interior surfaces.

SUMMARY OF THE INVENTION

A method of fabricating a silicon-on-insulator structure having asilicon surface layer in a semiconductor workpiece, is carried out bymaintaining the workpiece at an elevated temperature and producing anoxygen-containing plasma in the chamber while applying a bias to theworkpiece and setting the bias to a level corresponding to an implantdepth in the workpiece below the silicon surface layer to which oxygenatoms are to be implanted, whereby to form an oxygen-implanted layer inthe workpiece having an oxygen concentration distribution generallycentered at the implant depth and having a finite oxygen concentrationin the silicon surface layer. The oxygen concentration in the siliconsurface layer is then reduced to permit epitaxial silicon deposition.

The step of reducing the oxygen concentration in the silicon surfacelayer can include enriching the silicon content of the silicon surfacelayer by implanting silicon atoms into the silicon surface layer,implanting an oxygen-getter species into the silicon surface layerand/or ion implanting a deep damage layer below the implant depth of theimplanted oxygen. The workpiece is heated so as to cause the implantedoxygen to migrate downwardly to the deep damage layer, the implantedsilicon to become substitutional in the silicon surface layer and forimplanted hydrogen to bond with oxygen in the silicon surface layer andthe resulting water molecules to evaporate out of the silicon surfacelayer.

The plasma may be generated by causing a plasma current to oscillate ina circulatory reentrant path through an external conduit and theprocessing zone of the chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a first case that maintains an overhead torroidalplasma current path.

FIG. 2 is a side view of a case corresponding to the case of FIG. 1.

FIG. 3 is a graph illustrating the behavior of free fluorineconcentration in the plasma with variations in wafer-to-ceiling gapdistance.

FIG. 4 is a graph illustrating the behavior of free fluorineconcentration in the plasma with variations in RF bias power applied tothe workpiece.

FIG. 5 is a graph illustrating the behavior of free fluorineconcentration in the plasma with variations in RF source power appliedto the coil antenna.

FIG. 6 is a graph illustrating the behavior of free fluorineconcentration in the plasma with variations in reactor chamber pressure.

FIG. 7 is a graph illustrating the behavior of free fluorineconcentration in the plasma with variations in partial pressure of aninert diluent gas such as Argon.

FIG. 8 is a graph illustrating the degree of dissociation of process gasas a function of source power for an inductively coupled reactor and fora reactor according to an embodiment of the present invention.

FIG. 9 illustrates a variation of the case of FIG. 1.

FIGS. 10 and 11 illustrate a variation of the case of FIG. 1 in which aclosed magnetic core is employed.

FIG. 12 illustrates another case of the invention in which a torroidalplasma current path passes beneath the reactor chamber.

FIG. 13 illustrates a variation of the case of FIG. 10 in which plasmasource power is applied to a coil wound around a distal portion theclosed magnetic core.

FIG. 14 illustrates a case that establishes two parallel torroidalplasma currents.

FIG. 15 illustrates a case that establishes a plurality of individuallycontrolled parallel torroidal plasma currents.

FIG. 16 illustrates a variation of the case of FIG. 15 in which theparallel torroidal plasma currents enter and exit the plasma chamberthrough the vertical side wall rather than the ceiling.

FIG. 17A illustrates a case that maintains a pair of mutually orthogonaltorroidal plasma currents across the surface of the workpiece.

FIG. 17B illustrates the use of plural radial vanes in the case of FIG.17A.

FIGS. 18 and 19 illustrate an case of the invention in which thetorroidal plasma current is a broad belt that extends across a wide pathsuitable for processing large wafers.

FIG. 20 illustrates a variation of the case of FIG. 18 in which anexternal section of the torroidal plasma current path is constricted.

FIG. 21 illustrates a variation of the case of FIG. 18 employingcylindrical magnetic cores whose axial positions may be adjusted toadjust ion density distribution across the wafer surface.

FIG. 22 illustrates a variation of FIG. 21 in which a pair of windingsare wound around a pair of groups of cylindrical magnetic cores.

FIG. 23 illustrates a variation of FIG. 22 in which a single commonwinding is wound around both groups of cores.

FIGS. 24 and 25 illustrate an case that maintains a pair of mutuallyorthogonal torroidal plasma currents which are wide belts suitable forprocessing large wafers.

FIG. 26 illustrates a variation of the case of FIG. 25 in which magneticcores are employed to enhance inductive coupling.

FIG. 27 illustrates a modification of the case of FIG. 24 in which theorthogonal plasma belts enter and exit the reactor chamber through thevertical side wall rather than through the horizontal ceiling.

FIG. 28A illustrates an implementation of the case of FIG. 24 whichproduces a rotating torroidal plasma current.

FIG. 28B illustrates a version of the case of FIG. 28A that includesmagnetic cores.

FIG. 29 illustrates a preferred case of the invention in which acontinuous circular plenum is provided to enclose the torroidal plasmacurrent.

FIG. 30 is a top sectional view corresponding to FIG. 29.

FIGS. 31A and 31B are front and side sectional views corresponding toFIG. 30.

FIG. 32 illustrates a variation of the case 29 employing threeindependently driven RF coils underneath the continuous plenum facing at120-degree intervals.

FIG. 33 illustrates a variation of the case of FIG. 32 in which thethree RF coils are driven at 120-degree phase to provide an azimuthallyrotating plasma.

FIG. 34 illustrates a variation of the case of FIG. 33 in which RF drivecoils are wound around vertical external ends of respective magneticcores whose opposite ends extend horizontally under the plenum atsymmetrically distributed angles.

FIG. 35 is a version of the case of FIG. 17 in which the mutuallytransverse hollow conduits are narrowed as in the case of FIG. 20.

FIG. 36 is a version of the case of FIG. 24 but employing a pair ofmagnetic cores 3610, 3620 with respective windings 3630, 3640therearound for connection to respective RF power sources.

FIG. 37 is a case corresponding to that of FIG. 35 but having threeinstead of two reentrant conduits with a total of six reentrant ports tothe chamber.

FIG. 38 is a case corresponding to that of FIG. 38 but having threeinstead of two reentrant conduits with a total of six reentrant ports tothe chamber.

FIG. 39 is a case corresponding to that of FIG. 35 in which the externalconduits join together in a common plenum 3910.

FIG. 40 is a case corresponding to that of FIG. 36 in which the externalconduits join together in a common plenum 4010.

FIG. 41 is a case corresponding to that of FIG. 37 in which the externalconduits join together in a common plenum 4110.

FIG. 42 is a case corresponding to that of FIG. 38 in which the externalconduits join together in a common plenum 4210.

FIG. 43 is a case corresponding to that of FIG. 17 in which the externalconduits join together in a common plenum 4310.

FIG. 44 illustrates cases a reactor similar to that of FIG. 1 and havinga magnetic pole piece for controlling plasma ion density uniformity.

FIG. 45 illustrates a reactor like that of FIG. 44 in which the magneticpole piece has a reduced diameter near the ceiling surface, and theceiling is a dual zone gas distribution plate.

FIGS. 46, 47 and 48 illustrate different shapes for the pole piece.

FIG. 49 illustrates one implementation of the gas distribution plate.

FIG. 50 is a detailed view of a gas injection orifice in FIG. 49.

FIG. 51 is a graph depicting the magnetic field that the magnetic polepiece can generate.

FIG. 52 is a graph of the magnetic field magnitude as a function ofradius.

FIGS. 53 and 54 illustrate different ways of controlling process gasflow.

FIGS. 55A and 55B illustrate the use of a splitter in the torroidalplasma path.

FIGS. 56A, 56B and 56C illustrate use of splitters where the torroidalplasma current enters the chamber vertically.

FIGS. 57 and 58 illustrate different shapes for a splitter.

FIGS. 59A and 59B illustrate use of splitters where the torroidal plasmacurrent enters the chamber radially.

FIGS. 60, 61, 62 and 63 illustrate the use of splitters where thetorroidal plasma current is introduced vertically at a corner of thechamber.

FIG. 64 illustrates how a splitter may extend only part of the processregion height.

FIGS. 65A, 65B and 66 illustrate a splitter design adapted to increasethe effective radial path length of the torroidal plasma current insidethe chamber for a given chamber diameter.

FIG. 67 illustrates the use of MERIE magnets with the torroidal plasmacurrent source of FIG. 1.

FIGS. 68 and 69 illustrate the use of fins to better confine thetorroidal plasma current to the processing region.

FIGS. 70, 71A, and 71B illustrate an RF power applicator havingdistributed inductances.

FIG. 72 illustrates distributed inductances corresponding to the FIGS.70, 71A and 71B.

FIG. 73 illustrates a circular arrangement of the distributedinductances of FIG. 72.

FIG. 74 illustrates distributed inductances and capacitances in anarrangement corresponding to that of FIGS. 71A and 71B.

FIGS. 75 and 76 are schematic diagrams illustrating different ways ofinductively coupling RF power using the magnetic core of FIGS. 71A and71B.

FIG. 77 illustrates the use of an insulator layer to electricallyisolate the termination sections and torroidal tubes of FIG. 44.

FIG. 78 illustrates how the uniformity control magnet or magnetic polemay be placed under the wafer support pedestal.

FIG. 79 depicts an inductively coupled plasma immersion ion implantationreactor having an RF bias power applicator.

FIGS. 80A, 80B and 80C illustrate, respectively, an applied pulsed D.C.bias voltage, the corresponding sheath voltage behavior and an appliedRF bias voltage.

FIGS. 81A, 81B, 81C and 81D illustrate, respectively, an energydistribution of ion flux, a cycle of applied RF bias voltage, ionsaturation current as a function of D.C. bias voltage, and energydistribution of ion flux for different frequencies of RF bias voltage.

FIGS. 82A and 82B illustrate the temporal relationship between the poweroutput waveforms of the source power generator and the bias powergenerator in a push-pull mode.

FIGS. 82C and 82D illustrate the temporal relationship between the poweroutput waveforms of the source power generator and the bias powergenerator in an in-synchronism mode.

FIGS. 82E and 82F illustrate the temporal relationship between the poweroutput waveforms of the source power generator and the bias powergenerator in a symmetric mode.

FIGS. 82G and 82H illustrate the temporal relationship between the poweroutput waveforms of the source power generator and the bias powergenerator in a non-symmetric mode.

FIGS. 83A and 83B illustrate different versions of a capacitivelycoupled plasma immersion ion implantation reactor having an RF biaspower applicator.

FIG. 84 illustrates a plasma immersion ion implantation reactor having areentrant torroidal path plasma source.

FIG. 85 illustrates a plasma immersion ion implantation reactor having atorroidal plasma source with two intersecting reentrant plasma paths.

FIG. 86 illustrates an interior surface of the ceiling of the reactor ofFIG. 85.

FIG. 87 illustrates a gas distribution panel of the reactor of FIG. 85.

FIG. 88 is a partial view of the reactor of FIG. 85 modified to includea plasma control center electromagnet.

FIGS. 89A and 89B are side and top views, respectively, of a version ofthe reactor of FIG. 88 having, in addition, a plasma control outerelectromagnet.

FIGS. 90A, 90B and 90C are cross-sectional side view of the outerelectromagnet of FIG. 89A with different gap distances of a bottom platefor regulating magnetic flux.

FIG. 91 illustrates an RF bias power coupling circuit in the reactor ofFIG. 85.

FIG. 92 depicts an RF bias voltage waveform in accordance with a biasvoltage control feature.

FIG. 93 is a block diagram illustrating a control system for controllingbias voltage in accordance with the feature illustrated in FIG. 92.

FIG. 94 is a top view of a vacuum control valve employed in the reactorof FIG. 85.

FIG. 95 is a cross-sectional side view of the valve of FIG. 94 in theclosed position.

FIG. 96 is a side view of the interior surface of the housing of thevalve of FIG. 95 with an orientation at right angles to that of FIG. 95.

FIG. 97 is a cross-sectional side view of a high voltage wafer supportpedestal useful in the reactor of FIG. 85.

FIG. 98 is an enlarged cross-sectional view of the wafer supportpedestal of FIG. 97 illustrating a fastener therein.

FIG. 99 is a block diagram illustrating an ion implantation processingsystem including a plasma immersion ion implantation reactor.

FIG. 100 is a graph illustrating electron density as a function ofapplied plasma source power for the inductively coupled plasma immersionion implantation reactor of FIG. 79 and the torroidal source plasmaimmersion ion implantation reactor of FIG. 85.

FIG. 101 is a graph illustrating free fluorine density as a function ofapplied plasma source power for the inductively coupled plasma immersionion implantation reactor of FIG. 79 and the torroidal source plasmaimmersion ion implantation reactor of FIG. 85.

FIG. 102 is a graph illustrating electron density as a function ofapplied plasma source power for the capacitively coupled plasmaimmersion ion implantation reactor of FIG. 83A and the torroidal sourceplasma immersion ion implantation reactor of FIG. 85.

FIG. 103 is a graph illustrating dopant concentration as a function ofjunction depth for different ion energies in the reactor of FIG. 85 andin a convention ion beam implant machine.

FIG. 104 is a graph illustrating dopant concentration before and afterpost-implant rapid thermal annealing.

FIG. 105 is a graph illustrating dopant concentration before and afterdynamic surface annealing in the torroidal source plasma immersion ionimplantation reactor of FIG. 85 and in a convention ion beam implantmachine.

FIG. 106 is a graph depicting wafer resistivity after ion implantationand annealing as a function of junction depth obtained with the reactorof FIG. 85 using dynamic surface annealing and with a conventional ionbeam implant machine using rapid thermal annealing.

FIG. 107 is a graph depicting implanted dopant concentration obtainedwith the reactor of FIG. 85 before and after dynamic surface annealing.

FIG. 108 is a graph of RF bias voltage in the reactor of FIG. 85 (leftordinate) and of beamline voltage in a beamline implant machine (rightordinate) as a function of junction depth.

FIG. 109 is a cross-sectional view of the surface of a wafer during ionimplantation of source and drain contacts and of the polysilicon gate ofa transistor.

FIG. 110 is a cross-sectional view of the surface of a wafer during ionimplantation of the source and drain extensions of a transistor.

FIG. 111 is a flow diagram illustrating an ion implantation processcarried out using the reactor of FIG. 85.

FIG. 112 is a flow diagram illustrating a sequence of possiblepre-implant, ion implant and possible post implant processes carriedusing the reactor of FIG. 85 in the system of FIG. 99.

FIG. 113 is a block diagram illustrating a process for forming an SOIstructure.

FIG. 114 is a block diagram illustrating individual steps for carryingout one aspect of the process of FIG. 113.

FIG. 115 is a block diagram illustrating another process for forming anSOI structure.

FIG. 116 is a block diagram illustrating individual steps for carryingout one aspect of the process of FIG. 114.

FIG. 117A is a partial cross-sectional view of a semiconductor substratein which an SOI structure is to be formed.

FIG. 117B is a graph corresponding to FIG. 117A illustrating thedistribution of implanted oxygen atoms with reference to the process ofFIG. 113.

FIG. 117C is a graph corresponding to FIG. 117A illustrating thedistribution of implanted oxygen atoms and of implanted atoms (dashedline) creating a damage layer.

FIG. 117D is a graph corresponding to FIG. 117A illustrating thedistribution of implanted oxygen atoms after an annealing step duringwhich the implanted oxygen atoms migrate toward the damage layer.

FIG. 118A is another partial cross-sectional view of a semiconductorsubstrate in which an SOI structure is to be formed, and depicting aplasma process in which silicon and/or hydrogen is implanted in a thinsurface layer of the substrate during epitaxial deposition of silicon onthe substrate surface.

FIG. 118B is a diagram corresponding to FIG. 118A illustrating the depthdistribution of silicon atoms in the plasma process of FIG. 118A.

FIG. 119 is a block diagram illustrating a plasma process forfabricating an SOI structure in which silicon implantation and silicondeposition are performed separately.

FIGS. 120A through 120F illustrate the layered structure of asemiconductor substrate at respective stages of the process of FIG. 119.

FIG. 121 is a block diagram illustrating a first modification of theplasma process of FIG. 119.

FIG. 122 is a block diagram illustrating a second modification of theplasma process of FIG. 119.

FIG. 123 illustrates a modification of the torroidal source reactorincorporating radiant and conductive heating devices for heating theworkpiece or wafer, a wafer temperature probe, and a pulsed D.C. biasvoltage source for the wafer support pedestal.

FIG. 124 is a block diagram of a system including a torroidal sourceplasma immersion ion implantation reactor for fabricating an SOIstructure.

FIG. 125 is a block diagram of a modified version of the system of FIG.124.

DETAILED DESCRIPTION OF THE INVENTION

Description of a Torroidal Source Reactor:

Referring to FIG. 1, a plasma reactor chamber 100 enclosed by acylindrical side wall 105 and a ceiling 110 houses a wafer pedestal 115for supporting a semiconductor wafer or workpiece 120. A process gassupply 125 furnishes process gas into the chamber 100 through gas inletnozzles 130 a-130 d extending through the side wall 105. A vacuum pump135 controls the pressure within the chamber 100, typically holding thepressure below 0.5 milliTorr (mT). A half-torroidal hollow tubeenclosure or conduit 150 extends above the ceiling 110 in a half circle.The conduit 150, although extending externally outwardly from ceiling110, is nevertheless part of the reactor and forms a wall of thechamber. Internally it shares the same evacuated atmosphere as existselsewhere in the reactor. In fact, the vacuum pump 135, instead of beingcoupled to the bottom of the main part of the chamber as illustrated inFIG. 1, may instead be coupled to the conduit 150. The conduit 150 hasone open end 150 a sealed around a first opening 155 in the reactorceiling 110 and its other end 150 b sealed around a second opening 160in the reactor ceiling 110. The two openings or ports 150, 160 arelocated on generally opposite sides of the wafer support pedestal 115.The hollow conduit 150 is reentrant in that it provides a flow pathwhich exits the main portion of the chamber at one opening and re-entersat the other opening. In this specification, the conduit 150 may bedescribed as being half-torroidal, in that the conduit is hollow andprovides a portion of a closed path in which plasma may flow, the entirepath being completed by flowing across the entire process regionoverlying the wafer support pedestal 115. Notwithstanding the use of theterm torroidal, the trajectory of the path as well as thecross-sectional shape of the path or conduit 150 may be circular ornon-circular, and may be square, rectangular or any other shape either aregular shape or irregular.

The external conduit 150 may be formed of a relatively thin conductorsuch as sheet metal, but sufficiently strong to withstand the vacuumwithin the chamber. In order to suppress eddy currents in the sheetmetal of the hollow conduit 150 (and thereby facilitate coupling of anRF inductive field into the interior of the conduit 150), an insulatinggap 152 extends across and through the hollow conduit 150 so as toseparate it into two tubular sections. The gap 152 is filled by a ring154 of insulating material such as a ceramic in lieu of the sheet metalskin, so that the gap is vacuum tight. A second insulating gap 153 maybe provided, so that one section of the conduit 150 is electricallyfloating. A bias RF generator 162 applies RF bias power to the waferpedestal 115 and wafer 120 through an impedance match element 164.

The hollow conduit 150 may be formed of a machined metal, such asaluminum or aluminum alloy. Passages for liquid cooling or heating maybe incorporated in the walls of the hollow conduit.

Alternatively, the hollow conduit 150 may be formed of a non-conductivematerial instead of the conductive sheet metal. The non-conductivematerial may be a ceramic, for example. In such an alternative case,neither gap 152 or 153 is required.

An antenna 170 such as a winding or coil 165 disposed on one side of thehollow conduit 150 and wound around an axis parallel to the axis ofsymmetry of the half-torroidal tube is connected through an impedancematch element 175 to an RF power source 180. The antenna 170 may furtherinclude a second winding 185 disposed on the opposite side of the hollowconduit 150 and wound in the same direction as the first winding 165 sothat the magnetic fields from both windings add constructively.

Process gases from the chamber 100 fill the hollow conduit 150. Inaddition, a separate process gas supply 190 may supply process gasesdirectly in to the hollow conduit 150 through a gas inlet 195. The RFfield in the external hollow conduit 150 ionizes the gases in the tubeto produce a plasma. The RF field induced by the circular coil antenna170 is such that the plasma formed in the tube 150 reaches through theregion between the wafer 120 and the ceiling 110 to complete a torroidalpath that includes the half-torroidal hollow conduit 150. As employedherein, the term torroidal refers to the closed and solid nature of thepath, but does not refer or limit its cross-sectional shape ortrajectory, either of which may be circular or non-circular or square orotherwise. Plasma circulates (oscillates) through the complete torroidalpath or region which may be thought of as a closed plasma circuit. Thetorroidal region extends across the diameter of the wafer 120 and, incertain cases, has a sufficient width in the plane of the wafer so thatit overlies the entire wafer surface.

The RF inductive field from the coil antenna 170 includes a magneticfield which itself is closed (as are all magnetic fields), and thereforeinduces a plasma current along the closed torroidal path described here.It is believed that power from the RF inductive field is absorbed atgenerally every location along the closed path, so that plasma ions aregenerated all along the path. The RF power absorption and rate of plasmaion generation may vary among different locations along the closed pathdepending upon a number of factors. However, the current is generallyuniform along the closed path length, although the current density mayvary. This current alternates at the frequency of the RF signal appliedto the antenna 170. However, since the current induced by the RFmagnetic field is closed, the current must be conserved around thecircuit of the closed path, so that the amount of current flowing in anyportion of the closed path is generally the same as in any other portionof the path. As will be described below, this fact is exploited in theinvention to great advantage.

The closed torroidal path through which the plasma current flows isbounded by plasma sheaths formed at the various conductive surfacesbounding the path. These conductive surfaces include the sheet metal ofthe hollow conduit 150, the wafer (and/or the wafer support pedestal)and the ceiling overlying the wafer. The plasma sheaths formed on theseconductive surfaces are charge-depleted regions produced as the resultof the charge imbalance due to the greater mobility of the low-massnegative electrons and the lesser mobility of the heavy-mass positiveions. Such a plasma sheath has an electric field perpendicular to thelocal surface underlying the sheath. Thus, the RF plasma current thatpasses through the process region overlying the wafer is constricted byand passes between the two electric fields perpendicular to the surfaceof the ceiling facing the wafer,and the surface of the wafer facing thegas distribution plate. The thickness of the sheath (with RF biasapplied to the workpiece or other electrode) is greater where theelectric field is concentrated over a small area, such as the wafer, andis less in other locations such as the sheath covering the ceiling andthe large adjoining chamber wall surfaces. Thus, the plasma sheathoverlying the wafer is much thicker. The electric fields of the waferand ceiling/gas distribution plate sheaths are generally parallel toeach other and perpendicular to the direction of the RF plasma currentflow in the process region.

When RF power is first applied to the coil antenna 170, a dischargeoccurs across the gap 152 to ignite a capacitively coupled plasma fromgases within the hollow conduit 150. Above a threshold power level, thedischarge and plasma current become spatially continuous through thelength of the hollow conduit 150 and along the entire torroidal path.Thereafter, as the plasma current through the hollow conduit 150increases, the inductive coupling of the RF field becomes more dominantso that the plasma becomes an inductively coupled plasma. Alternatively,plasma may be initiated by other means, such as by RF bias applied tothe workpiece support or other electrode or by a spark or ultravioletlight source.

In order to avoid edge effects at the wafer periphery, the ports 150,160 are separated by a distance that exceeds the diameter of the wafer.For example, for a 12 inch diameter wafer, the ports 150, 160 are about14 to 22 inches apart. For an 8 inch diameter wafer, the ports 150, 160are about 9 to 16 inches apart.

Notwithstanding the use of the term “wafer”, the workpiece may be anyshape, such as rectangular. The workpiece material may be asemiconductor, insulator, or conductor, or a combination of variousmaterials. The workpiece may have 2-dimensional or 3-dimensionalstructure, as well.

Advantages:

A significant advantage is that power from the RF inductive field isabsorbed throughout the relatively long closed torroidal path (i.e.,long relative to the gap length between the wafer and the reactorceiling), so that RF power absorption is distributed over a large area.As a result, the RF power density in the vicinity of thewafer-to-ceiling gap (i.e., the process region 121 best shown in FIG. 2,not to be confused with the insulating gap 152) is relatively low, thusreducing the likelihood of device damage from RF fields. In contrast, inprior inductively coupled reactors, all of the RF power is absorbedwithin the narrow wafer-to-ceiling gap, so that it is greatlyconcentrated in that region. Moreover, this fact often limits theability to narrow the wafer-to-ceiling gap (in the quest of otheradvantages) or, alternatively, requires greater concentration of RFpower in the region of the wafer. Thus, the invention overcomes alimitation of long standing in the art. This aspect enhances processperformance for some applications by reducing residency time of thereactive gases through a dramatic reduction in volume of the processregion or process zone overlying the wafer, as discussed previouslyherein.

A related and even more important advantage is that the plasma densityat the wafer surface can be dramatically increased without increasingthe RF power applied to the coil antenna 170 (leading to greaterefficiency). This is accomplished by reducing the cross-sectional areaof the torroidal path in the vicinity of the pedestal surface and wafer120 relative to the remainder of the torroidal path. By so constrictingthe torroidal path of the plasma current near the wafer only, thedensity of the plasma near the wafer surface is increasedproportionately. This is because the torroidal path plasma currentthrough the hollow conduit 150 must be at least nearly the same as theplasma current through the pedestal-to-ceiling (wafer-to-ceiling) gap.

A significant difference over the prior art is that not only is the RFfield remote from the workpiece, and not only can ion density beincreased at the wafer surface without increasing the applied RF field,but the plasma ion density and/or the applied RF field may be increasedwithout increasing the minimum wafer-to-ceiling gap length. Formerly,such an increase in plasma density necessitated an increase in thewafer-to-ceiling gap to avoid strong fields at the wafer surface. Incontrast, in the present invention the enhanced plasma density isrealized without requiring any increase in the wafer-to-ceiling gap toavoid a concomitant increase in RF magnetic fields at the wafer surface.This is because the RF field is applied remotely from the wafer andmoreover need not be increased to realize an increase in plasma densityat the wafer surface. As a result, the wafer-to-ceiling gap can bereduced down to a fundamental limit to achieve numerous advantages. Forexample, if the ceiling surface above the wafer is conductive, thenreducing the wafer-to-ceiling gap improves the electrical or groundreference provided by the conductive ceiling surface. A fundamentallimit on the minimum wafer-to-ceiling gap length is the sum of thethicknesses of the plasma sheaths on the wafer surface and on theceiling surface.

A further advantage of the invention is that because the RF inductivefield is applied along the entire torroidal path of the RF plasmacurrent (so that its absorption is distributed as discussed above), thechamber ceiling 110, unlike with most other inductively poweredreactors, need not function as a window to an inductive field andtherefore may be formed of any desired material, such as a highlyconductive and thick metal, and therefore may comprise a conductive gasdistribution plate as will be described below, for example. As a result,the ceiling 110 readily provides a reliable electric potential or groundreference across the entire plane of the pedestal or wafer 120.

Increasing the Plasma Ion Density:

One way of realizing higher plasma density near the wafer surface byreducing plasma path cross-sectional area over the wafer is to reducethe wafer-to-ceiling gap length. This may be accomplished by simplyreducing the ceiling height or by introducing a conductive gasdistribution plate or showerhead over the wafer, as illustrated in FIG.2. The gas distribution showerhead 210 of FIG. 2 consists of a gasdistribution plenum 220 connected to the gas supply 125 andcommunicating with the process region over the wafer 120 through pluralgas nozzle openings 230. The advantage of the conductive showerhead 210is two-fold: First, by virtue of its close location to the wafer, itconstricts the plasma path over the wafer surface and thereby increasesthe density of the plasma current in that vicinity. Second, it providesa uniform electrical potential reference or ground plane close to andacross the entire wafer surface.

In order to avoid arcing across the openings 230, each opening 230 maybe relatively small, on the order of a millimeter (e.g., hole diameteris approximately 0.5 mm). The spacing between adjacent openings may beon the order of a several millimeters.

The conductive showerhead 210 constricts the plasma current path ratherthan providing a short circuit through itself because a plasma sheath isformed around the portion of the showerhead surface immersed in theplasma. The sheath has a greater impedance to the plasma current thanthe space between the wafer 120 and the showerhead 210, and thereforevirtually all the plasma current goes around the conductive showerhead210.

It is not necessary to employ a showerhead (e.g., the showerhead 210) inorder to constrict the torroidal plasma current or path in the vicinityof the process region overlying the wafer. The path constriction andconsequent increase in plasma ion density in the process region may beachieved without the showerhead 210 by similarly reducing thewafer-to-ceiling height. If the showerhead 210 is eliminated in thismanner, then the process gases may be supplied into the chamber interiorby means of conventional gas inlet nozzles, gas diffusers, or gas slots(not shown).

One advantage of the showerhead 210 is that different mixtures ofreactive and inert process gas ratios may be introduced throughdifferent orifices 230 at different radii, in order to finely adjust theuniformity of plasma effects on photoresist, for example. Thus, forexample, a greater proportion of inert gas to reactive gas may besupplied to the orifices 230 lying outside a median radius while agreater proportion of reactive gas to inert gas may be supplied to theorifices 230 within that median radius.

As will be described below, another way in which the torroidal plasmacurrent path may be constricted in the process region overlying thewafer (in order to increase plasma ion density over the wafer) is toincrease the plasma sheath thickness on the wafer by increasing the RFbias power applied to the wafer support pedestal. Since as describedpreviously the plasma current across the process region is confinedbetween the plasma sheath at the wafer surface and the plasma sheath atthe ceiling (or showerhead) surface, increasing the plasma sheaththickness at the wafer surface necessarily decreases the cross-sectionof the portion of the torroidal plasma current within process region,thereby increasing the plasma ion density in the process region. Thus,as will be described more fully later in this specification, as RF biaspower on the wafer support pedestal is increased, plasma ion densitynear the wafer surface is increased accordingly.

High Etch Selectivity at High Etch Rates:

The invention solves the problem of poor etch selectivity whichsometimes occurs with a high density plasma. The reactor of FIGS. 1 and2 has a silicon dioxide-to-photoresist etch selectivity as high as thatof a capacitively coupled plasma reactor (about 7:1) while providinghigh etch rates approaching that of a high density inductively coupledplasma reactor. It is believed that the reason for this success is thatthe reactor structure of FIGS. 1 and 2 reduces the degree ofdissociation of the reactive process gas, typically a fluorocarbon gas,so as to reduce the incidence of free fluorine in the plasma region overthe wafer 120. Thus, the proportion of free fluorine in the plasmarelative to other species dissociated from the fluorocarbon gas isdesirably reduced. Such other species include the protective carbon-richpolymer precursor species formed in the plasma from the fluorocarbonprocess gas and deposited on the photoresist as a protective polymercoating. They further include less reactive etchant species such as CFand CF₂ formed in the plasma from the fluorocarbon process gas. Freefluorine tends to attack photoresist and the protective polymer coatingformed thereover as vigorously as it attacks silicon dioxide, thusreducing oxide-to-photoresist etch selectivity. On the other hand, theless reactive etch species such as CF₂ or CF tend to attack photoresistand the protective polymer coating formed thereover more slowly andtherefore provide superior etch selectivity.

It is believed that the reduction in the dissociation of the plasmaspecies to free fluorine is accomplished in the invention by reducingthe residency time of the reactive gas in the plasma. This is becausethe more complex species initially dissociated in the plasma from thefluorocarbon process gas, such as CF₂ and CF are themselves ultimatelydissociated into simpler species including free fluorine, the extent ofthis final step of dissociation depending upon the residency time of thegas in the plasma. The term “residency time” or “residence time” asemployed in this specification corresponds generally to the average timethat a process gas molecule and the species dissociated from the thatmolecule are present in the process region overlying the workpiece orwafer. This time or duration extends from the initial injection of themolecule into the process region until the molecule and/or itsdissociated progeny are pass out of the process region along the closedtorroidal path described above that extends through the processing zone.

It is also believed that the reduction in the dissociation of the plasmaspecies to free fluorine is accomplished by reducing the power densityof the applied plasma source power as compared to conventionalinductively coupled plasma sources. As stated above, power from the RFinductive field is absorbed throughout the relatively long closedtorroidal path (i.e., long relative to the gap length between the waferand the reactor ceiling), so that RF power absorption is distributedover a large area. As a result, the RF power density in the vicinity ofthe wafer-to-ceiling gap (i.e., the process region 121 best shown inFIG. 2, not to be confused with the insulating gap 152) is relativelylow, thus reducing the dissociation of molecular gases.

As stated above, the invention enhances etch selectivity by reducing theresidency time in the process region of the fluorocarbon process gas.The reduction in residency time is achieved by constricting the plasmavolume between the wafer 120 and the ceiling 110.

The reduction in the wafer-to-ceiling gap or volume has certainbeneficial effects. First, it increases plasma density over the wafer,enhancing etch rate. Second, residency time falls as the volume isdecreased. As referred to above, the small volume is made possible inthe present invention because, unlike conventional inductively coupledreactors, the RF source power is not deposited within the confines ofthe process region overlying the wafer but rather power deposition isdistributed along the entire closed torroidal path of the plasmacurrent. Therefore, the wafer-to-ceiling gap can be less than a skindepth of the RF inductive field, and in fact can be so small as tosignificantly reduce the residency time of the reactive gases introducedinto the process region, a significant advantage.

There are two ways of reducing the plasma path cross-section andtherefore the volume over the wafer 120. One is to reduce thewafer-to-showerhead gap distance. The other is to increase the plasmasheath thickness over the wafer by increasing the bias RF power appliedto the wafer pedestal 115 by the RF bias power generator 162, as brieflymentioned above. Either method results in a reduction in free fluorinecontent of the plasma in the vicinity of the wafer 120 (and consequentincrease in dielectric-to-photoresist etch selectivity) as observedusing optical emission spectroscopy (OES) techniques.

There are three additional methods of the invention for reducing freefluorine content to improve etch selectivity. One method is to introducea non-chemically reactive diluent gas such as argon into the plasma. Theargon gas may be introduced outside and above the process region byinjecting it directly into the hollow conduit 150 from the secondprocess gas supply 190, while the chemically reactive process gases(fluorocarbon gases) enter the chamber only through the showerhead 210.With this advantageous arrangement, the argon ions, neutrals, andexcited neutrals propagate within the torroidal path plasma current andthrough the process region across the wafer surface to dilute the newlyintroduced reactive (e.g., fluorocarbon) gases and thereby effectivelyreduce their residency time over the wafer. Another method of reducingplasma free fluorine content is to reduce the chamber pressure. Afurther method is to reduce the RF source power applied to the coilantenna 170.

FIG. 3 is a graph illustrating a trend observed in the invention inwhich the free fluorine content of the plasma decreases as thewafer-to-showerhead gap distance is decreased. FIG. 4 is a graphillustrating that the free fluorine content of the plasma is decreasedby decreasing the plasma bias power applied to the wafer pedestal 115.FIG. 5 is a graph illustrating that plasma free fluorine content isreduced by reducing the RF source power applied to the coil antenna 170.FIG. 6 is a graph illustrating that the free fluorine content is reducedby reducing chamber pressure. FIG. 7 is a graph illustrating that plasmafree fluorine content is reduced by increasing the diluent (Argon gas)flow rate into the tubular enclosure 150. The graphs of FIGS. 3-7 aremerely illustrative of plasma behavioral trends inferred from numerousOES observations and do not depict actual data.

Wide Process Window:

The chamber pressure is generally less than 0.5 T and can be as low as 1mT. The process gas may be C₄F₈ injected into the chamber 100 throughthe gas distribution showerhead at a flow rate of about 15 cc/m with 150cc/m of Argon, with the chamber pressure being maintained at about 20mT. Alternatively, the Argon gas flow rate may be increased to 650 cc/mand the chamber pressure to 60 mT. The antenna 170 may be excited withabout 500 Watts of RF power at 13 MHz. The wafer-to-showerhead gap maybe about 0.3 inches to 2 inches. The bias RF power applied to the waferpedestal may be 13 MHz at 2000 Watts. Other selections of frequency maybe made. The source power applied to the coil antenna 170 may be as lowas 50 kHz or as high as several times 13 MHz or higher. The same is trueof the bias power applied to the wafer pedestal.

The process window for the reactor of FIGS. 1 and 2 is far wider thanthe process window for a conventional inductively coupled reactor. Thisis illustrated in the graph of FIG. 8, showing the specific neutral fluxof free fluorine as a function of RF source power for a conventionalinductive reactor and for the reactor of FIGS. 1 and 2. For theconventional inductively coupled reactor, FIG. 8 shows that the freefluorine specific flux begins to rapidly increase as the source powerexceeds between 50 and 100 Watts. In contrast, the reactor of FIGS. 1and 2 can accept source power levels approaching 1000 Watts before thefree fluorine specific flux begins to increase rapidly. Therefore, thesource power process window in the invention is nearly an order ofmagnitude wider than that of a conventional inductively coupled reactor,a significant advantage.

Dual Advantages:

The constriction of the torroidal plasma current path in the vicinity ofthe wafer or workpiece produces two independent advantages without anysignificant tradeoffs of other performance criteria: (1) the plasmadensity over the wafer is increased without requiring any increase inplasma source power, and (2) the etch selectivity to photoresist orother materials is increased, as explained above. It is believed that inprior plasma reactors it has been impractical if not impossible toincrease the plasma ion density by the same step that increases etchselectivity. Thus, the dual advantages realized with the torroidalplasma source of the present invention appear to be a revolutionarydeparture from the prior art.

Other Embodiments:

FIG. 9 illustrates a modification of the case of FIG. 1 in which theside antenna 170 is replaced by a smaller antenna 910 that fits insidethe empty space between the ceiling 110 and the hollow conduit 150. Theantenna 910 is a single coil winding centered with respect to the hollowconduit 150.

FIGS. 10 and 11 illustrate how the case of FIG. 1 may be enhanced by theaddition of a closed magnetically permeable core 1015 that extendsthrough the space between the ceiling 110 and the hollow conduit 150.The core 1015 improves the inductive coupling from the antenna 170 tothe plasma inside the hollow conduit 150.

Impedance match may be achieved without the impedance match circuit 175by using, instead, a secondary winding 1120 around the core 1015connected across a tuning capacitor 1130. The capacitance of the tuningcapacitor 1130 is selected to resonate the secondary winding 1120 at thefrequency of the RF power source 180. For a fixed tuning capacitor 1130,dynamic impedance matching may be provided by frequency tuning and/or byforward power servoing.

FIG. 12 illustrates a case of the invention in which a hollow tubeenclosure 1250 extends around the bottom of the reactor and communicateswith the interior of the chamber through a pair of openings 1260, 1265in the bottom floor of the chamber. A coil antenna 1270 follows alongside the torroidal path provided by the hollow tube enclosure 1250 inthe manner of the case of FIG. 1. While FIG. 12 shows the vacuum pump135 coupled to the bottom of the main chamber, it may just as well becoupled instead to the underlying conduit 1250.

FIG. 13 illustrates a variation of the case of FIGS. 10 and 11, in whichthe antenna 170 is replaced by an inductive winding 1320 surrounding anupper section of the core 1015. Conveniently, the winding 1320 surroundsa section of the core 1015 that is above the conduit 150 (rather thanbelow it). However, the winding 1320 can surround any section of thecore 1015.

FIG. 14 illustrates an extension of the concept of FIG. 13 in which asecond hollow tube enclosure 1450 runs parallel to the first hollowconduit 150 and provides a parallel torroidal path for a secondtorroidal plasma current. The tube enclosure 1450 communicates with thechamber interior at each of its ends through respective openings in theceiling 110. A magnetic core 1470 extends under both tube enclosures150, 1450 and through the coil antenna 170.

FIG. 15 illustrates an extension of the concept of FIG. 14 in which anarray of parallel hollow tube enclosures 1250 a, 1250 b, 1250 c, 1250 dprovide plural torroidal plasma current paths through the reactorchamber. In the case of FIG. 15, the plasma ion density is controlledindependently in each individual hollow conduit 1250 a-d by anindividual coil antenna 170 a-d, respectively, driven by an independentRF power source 180 a-d, respectively. Individual cylindrical open cores1520 a-1520 d may be separately inserted within the respective coilantennas 170 a-d. In this case, the relative center-to-edge ion densitydistribution may be adjusted by separately adjusting the power levels ofthe individual RF power sources 180 a-d.

FIG. 16 illustrates a modification of the case of FIG. 15 in which thearray of tube enclosures 1250 a-d extend through the side wall of thereactor rather than through the ceiling 110. Another modificationillustrated in FIG. 16 is the use of a single common magnetic core 1470adjacent all of the tube enclosures 1250 a-d and having the antenna 170wrapped around it so that a single RF source excites the plasma in allof the tube enclosures 1250 a-d.

FIG. 17A illustrates a pair of orthogonal tube enclosures 150-1 and150-2 extending through respective ports in the ceiling 110 and excitedby respective coil antennas 170-1 and 170-2. Individual cores 1015-1 and1015-2 are within the respective coil antennas 170-1 and 170-2. Thiscase creates two mutually orthogonal torroidal plasma current paths overthe wafer 120 for enhanced uniformity. The two orthogonal torroidal orclosed paths are separate and independently powered as illustrated, butintersect in the process region overlying the wafer, and otherwise donot interact. In order to assure separate control of the plasma sourcepower applied to each one of the orthogonal paths, the frequency of therespective RF generators 180 a, 180 b of FIG. 17 are different, so thatthe operation of the impedance match circuits 175 a, 175 b is decoupled.For example, the RF generator 180 a may produce an RF signal at 11 MHzwhile the RF generator 180 b may produce an RF signal at 12 MHz.Alternatively, independent operation may be achieved by offsetting thephases of the two RF generators 180 a, 180 b.

FIG. 17B illustrates how radial vanes 181 may be employed to guide thetorroidal plasma currents of each of the two conduits 150-1, 150-2through the processing region overlying the wafer support. The radialvanes 181 extend between the openings of each conduit near the sides ofthe chamber up to the edge of the wafer support. The radial vanes 181prevent diversion of plasma from one torroidal path to the othertorroidal path, so that the two plasma currents only intersect withinthe processing region overlying the wafer support.

Cases Suitable for Large Diameter Wafers:

In addition to the recent industry trends toward smaller device sizesand higher device densities, another trend is toward greater waferdiameters. For example, 12-inch diameter wafers are currently enteringproduction, and perhaps larger diameter wafers will be in the future.The advantage is greater throughput because of the large number ofintegrated circuit die per wafer. The disadvantage is that in plasmaprocessing it is more difficult to maintain a uniform plasma across alarge diameter wafer. The following cases of the present invention areparticularly adapted for providing a uniform plasma ion densitydistribution across the entire surface of a large diameter wafer, suchas a 12-inch diameter wafer.

FIGS. 18 and 19 illustrate a hollow tube enclosure 1810 which is a wideflattened rectangular version 1850 of the hollow conduit 150 of FIG. 1that includes an insulating gap 1852. This version produces a wide“belt” of plasma that is better suited for uniformly covering a largediameter wafer such as a 12-inch diameter wafer or workpiece. The widthW of the tube enclosure and of the pair of openings 1860, 1862 in theceiling 110 may exceed the wafer by about 5% or more. For example, ifthe wafer diameter is 10 inches, then the width W of the rectangulartube enclosure 1850 and of the openings 1860, 1862 is about 11 inches.FIG. 20 illustrates a modified version 1850′ of the rectangular tubeenclosure 1850 of FIGS. 18 and 19 in which a portion 1864 of theexterior tube enclosure 1850 is constricted.

FIG. 20 further illustrates the optional use of focusing magnets 1870 atthe transitions between the constricted and unconstricted portions ofthe enclosure 1850. The focusing magnets 1870 promote a better movementof the plasma between the constricted and unconstricted portions of theenclosure 1850, and specifically promote a more uniform spreading out ofthe plasma as it moves across the transition between the constrictedportion 1864 and the unconstricted portion of the tube enclosure 1850.

FIG. 21 illustrates how plural cylindrical magnetic cores 2110 may beinserted through the exterior region 2120 circumscribed by the tubeenclosure 1850. The cylindrical cores 2110 are generally parallel to theaxis of symmetry of the tube enclosure 1850. FIG. 22 illustrates amodification of the case of FIG. 21 in which the cores 2110 extendcompletely through the exterior region 2120 surrounded by the tubeenclosure 1850 are replaced by pairs of shortened cores 2210, 2220 inrespective halves of the exterior region 2120. The side coils 165, 185are replaced by a pair of coil windings 2230, 2240 surrounding therespective core pairs 2210, 2220. In this case, the displacement Dbetween the core pairs 2210, 2220 may be changed to adjust the iondensity near the wafer center relative to the ion density at the wafercircumference. A wider displacement D reduces the inductive couplingnear the wafer center and therefore reduces the plasma ion density atthe wafer center. Thus, an additional control element is provided forprecisely adjusting ion density spatial distribution across the wafersurface. FIG. 23 illustrates a variation of the case of FIG. 22 in whichthe separate windings 2230, 2240 are replaced by a single center winding2310 centered with respect to the core pairs 2210, 2220.

FIGS. 24 and 25 illustrate a case providing even greater uniformity ofplasma ion density distribution across the wafer surface. In the case ofFIGS. 24 and 25, two torroidal plasma current paths are established thatare transverse to one another and are mutually orthogonal. This isaccomplished by providing a second wide rectangular hollow enclosure2420 extending transversely and orthogonally relative to the first tubeenclosure 1850. The second tube enclosure 2420 communicates with thechamber interior through a pair of openings 2430, 2440 through theceiling 110 and includes an insulating gap 2452. A pair of side coilwindings 2450, 2460 along the sides of the second tube enclosure 2420maintain a plasma therein and are driven by a second RF power supply2470 through an impedance match circuit 2480. As indicated in FIG. 24,the two orthogonal plasma currents coincide over the wafer surface andprovide more uniform coverage of plasma over the wafer surface. Thiscase is expected to find particularly advantageous use for processinglarge wafers of diameters of 10 inches and greater.

As in the case of FIG. 17, the case of FIG. 24 creates two mutuallyorthogonal torroidal plasma current paths over the wafer 120 forenhanced uniformity. The two orthogonal torroidal or closed paths areseparate and independently powered as illustrated, but intersect in theprocess region overlying the wafer, and otherwise do not interact orotherwise divert or diffuse one another. In order to assure separatecontrol of the plasma source power applied to each one of the orthogonalpaths, the frequency of the respective RF generators 180, 2470 of FIG.24 are different, so that the operation of the impedance match circuits175, 2480 is decoupled. For example, the RF generator 180 may produce anRF signal at 11 MHz while the RF generator 2470 may produce an RF signalat 12 MHz. Alternatively, independent operation may be achieved byoffsetting the phases of the two RF generators 180, 2470.

FIG. 26 illustrates a variation of the case of FIG. 18 in which amodified rectangular enclosure 2650 that includes an insulating gap 2658communicates with the chamber interior through the chamber side wall 105rather than through the ceiling 110. For this purpose, the rectangularenclosure 2650 has a horizontal top section 2652, a pair of downwardlyextending legs 2654 at respective ends of the top section 2652 and apair of horizontal inwardly extending legs 2656 each extending from thebottom end of a respective one of the downwardly extending legs 2654 toa respective opening 2670, 2680 in the side wall 105.

FIG. 27 illustrates how a second rectangular tube enclosure 2710including an insulating gap 2752 may be added to the case of FIG. 26,the second tube enclosure 2710 being identical to the rectangular tubeenclosure 2650 of FIG. 26 except that the rectangular tube enclosures2650, 2710 are mutually orthogonal (or at least transverse to oneanother). The second rectangular tube enclosure communicates with thechamber interior through respective openings through the side wall 105,including the opening 2720. Like the case of FIG. 25, the tubeenclosures 2650 and 2710 produce mutually orthogonal torroidal plasmacurrents that coincide over the wafer surface to provide superioruniformity over a broader wafer diameter. Plasma source power is appliedto the interior of the tube enclosures through the respective pairs ofside coil windings 165, 185 and 2450, 2460.

FIG. 28A illustrates how the side coils 165, 185, 2450, 2460 may bereplaced (or supplemented) by a pair of mutually orthogonal interiorcoils 2820, 2840 lying within the external region 2860 surrounded by thetwo rectangular tube enclosures 2650, 2710. Each one of the coils 2820,2840 produces the torroidal plasma current in a corresponding one of therectangular tube enclosures 2650, 2710. The coils 2820, 2840 may bedriven completely independently at different frequencies or at the samefrequency with the same or a different phase. Or, they may be driven atthe same frequency but with a phase difference (i.e., 90 degrees) thatcauses the combined torroidal plasma current to rotate at the sourcepower frequency. In this case the coils 2820, 2840 are driven with thesin and cosine components, respectively, of a common signal generator2880, as indicated in FIG. 28A. The advantage is that the plasma currentpath rotates azimuthally across the wafer surface at a rotationalfrequency that exceeds the plasma ion frequency so that non-uniformitiesare better suppressed than in prior art methods such as MERIE reactorsin which the rotation is at a much lower frequency.

Referring now to FIG. 28B, radial adjustment of plasma ion density maybe generally provided by provision of a pair of magnetic cylindricalcores 2892, 2894 that may be axially moved toward or away from oneanother within the coil 2820 and a pair of magnetic cylindrical cores2896, 2898 that may be axially moved toward or away from one anotherwithin the coil 2840. As each pair of cores is moved toward one another,inductive coupling near the center of each of the orthogonal plasmacurrents is enhanced relative to the edge of the current, so that plasmadensity at the wafer center is generally enhanced. Thus, thecenter-to-edge plasma ion density may be controlled by moving the cores2892, 2894, 2896, 2898.

FIG. 29 illustrates an alternative case of the invention in which thetwo tube enclosures 2650, 2710 are merged together into a singleenclosure 2910 that extends 360 degrees around the center axis of thereactor that constitutes a single plenum. In the case of FIG. 29, theplenum 2910 has a half-dome lower wall 2920 and a half-dome upper wall2930 generally congruent with the lower wall 2920. The plenum 2910 istherefore the space between the upper and lower half-dome walls 2920,2930. An insulating gap 2921 may extend around the upper dome wall 2920and/or an insulating gap 2931 may extend around the lower dome wall2930. The plenum 2910 communicates with the chamber interior through anannular opening 2925 in the ceiling 110 that extends 360 degrees aroundthe axis of symmetry of the chamber.

The plenum 2910 completely encloses a region 2950 above the ceiling 110.In the case of FIG. 29, plasma source power is coupled into the interiorof the plenum 2910 by a pair of mutually orthogonal coils 2960, 2965.Access to the coils 2960, 2965 is provided through a vertical conduit2980 passing through the center of the plenum 2910. Preferably, thecoils 2960, 2965 are driven in quadrature as in the case of FIG. 28 toachieve an azimuthally circulating torroidal plasma current (i.e., aplasma current circulating within the plane of the wafer. The rotationfrequency is the frequency of the applied RF power. Alternatively, thecoils 2960, 2965 may be driven separately at different frequencies. FIG.30 is a top sectional view of the case of FIG. 29. FIGS. 31A and 31B arefront and side sectional views, respectively, corresponding to FIG. 30.

The pair of mutually orthogonal coils 2960, 2965 may be replaced by anynumber n of separately driven coils with their winding axes disposed at360/n degrees apart. For example, FIG. 32 illustrates the case where thetwo coils 2960, 2965 are replace by three coils 3210, 3220, 3230 withwinding axes placed at 120 degree intervals and driven by threerespective RF supplies 3240, 3250, 3260 through respective impedancematch circuits 3241, 3251, 3261. In order to produce a rotatingtorroidal plasma current, the three windings 3210, 3220, 3230 are driven120 degrees out of phase from a common power source 3310 as illustratedin FIG. 33. The cases of FIGS. 32 and 33 are preferred over the case ofFIG. 29 having only two coils, since it is felt much of the mutualcoupling between coils would be around rather than through the verticalconduit 2980.

FIG. 34 illustrates a case in which the three coils are outside of theenclosed region 2950, while their inductances are coupled into theenclosed region 2950 by respective vertical magnetic cores 3410extending through the conduit 2980. Each core 3410 has one end extendingabove the conduit 2980 around which a respective one of the coils 3210,3220, 3230 is wound. The bottom of each core 3410 is inside the enclosedregion 2950 and has a horizontal leg. The horizontal legs of the threecores 3410 are oriented at 120 degree intervals to provide inductivecoupling to the interior of the plenum 2910 similar to that provided bythe three coils inside the enclosed region as in FIG. 32.

The advantage of the flattened rectangular tube enclosures of the casesof FIGS. 18-28 is that the broad width and relatively low height of thetube enclosure forces the torroidal plasma current to be a wide thinbelt of plasma that more readily covers the entire surface of a largediameter wafer. The entirety of the tube enclosure need not be of themaximum width. Instead the outer section of the tube enclosure farthestfrom the chamber interior may be necked down, as discussed above withreference to the case of FIG. 20. In this case, it is preferable toprovide focusing magnets 1870 at the transition corners between the wideportion 1851 and the narrow section 1852 to force the plasma currentexiting the narrow portion 1852 to spread entirely across the entirewidth of the wide section 1851. If it is desired to maximize plasma iondensity at the wafer surface, then it is preferred that thecross-sectional area of the narrow portion 1852 be at least nearly asgreat as the cross-sectional area of the wide portion 1851. For example,the narrow portion 1852 may be a passageway whose height and width areabout the same while the wide portion 1851 may have a height that isless than its width.

The various cases described herein with air-core coils (i.e., coilswithout a magnetic core) may instead employ magnetic-cores, which can bethe open-magnetic-path type or the closed-magnetic-core type illustratedin the accompanying drawings. Furthermore, the various cases describedherein having two or more torroidal paths driven with different RFfrequencies may instead be driven with same frequency, and with the sameor different phases.

FIG. 35 is a version of the case of FIG. 17 in which the mutuallytransverse hollow conduits are narrowed as in the case of FIG. 20.

FIG. 36 is a version of the case of FIG. 24 but employing a pair ofmagnetic cores 3610, 3620 with respective windings 3630, 3640therearound for connection to respective RF power sources.

FIG. 37 is a case corresponding to that of FIG. 35 but having threeinstead of two reentrant conduits with a total of six reentrant ports tothe chamber. Having a number of symmetrically disposed conduits andreentrant ports greater than two (as in the case of FIG. 37) is believedto be particularly advantageous for processing wafers of diameter of 300mm and greater.

FIG. 38 is a case corresponding to that of FIG. 38 but having threeinstead of two reentrant conduits with a total of six reentrant ports tothe chamber.

FIG. 39 is a case corresponding to that of FIG. 35 in which the externalconduits join together in a common plenum 3910.

FIG. 40 is a case corresponding to that of FIG. 36 in which the externalconduits join together in a common plenum 4010.

FIG. 41 is a case corresponding to that of FIG. 37 in which the externalconduits join together in a common plenum 4110.

FIG. 42 is a case corresponding to that of FIG. 38 in which the externalconduits join together in a common plenum 4210.

FIG. 43 is a case corresponding to that of FIG. 17 in which the externalconduits join together in a common plenum 4310.

Advantageous Features:

Constricting the torroidal plasma current in the vicinity of the wafernot only improves etch selectivity but at the same time increases theetch rate by increasing the plasma ion density. It is believed no priorreactor has increased etch selectivity by the same mechanism thatincreases etch rate or plasma ion density over the workpiece.

Improving etch selectivity by constricting the torroidal plasma currentin the vicinity of the wafer or workpiece can be achieved in theinvention in any one of several ways. One way is to reduce thepedestal-to-ceiling or wafer-to-ceiling height. Another is to introducea gas distribution plate or showerhead over the wafer that constrictsthe path of the torroidal plasma ion current. Another way is to increasethe RF bias power applied to the wafer or workpiece. Any one or anycombination of the foregoing ways of improving etch selectivity may bechosen by the skilled worker in carrying out the invention.

Etch selectivity may be further improved in the invention by injectingthe reactive process gases locally (i.e., near the wafer or workpiece)while injecting an inert diluent gas (e.g., Argon) remotely (i.e., intothe conduit or plenum). This may be accomplished by providing a gasdistribution plate or showerhead directly over and facing the workpiecesupport and introducing the reactive process gas exclusively (or atleast predominantly) through the showerhead. Concurrently, the diluentgas is injected into the conduit well away from the process regionoverlying the wafer or workpiece. The torroidal plasma current thusbecomes not only a source of plasma ions for reactive ion etching ofmaterials on the wafer but, in addition, becomes an agent for sweepingaway the reactive process gas species and their plasma-dissociatedprogeny before the plasma-induced dissociation process is carried out tothe point of creating an undesirable amount of free fluorine. Thisreduction in the residence time of the reactive process gas speciesenhances the etch selectivity relative to photoresist and othermaterials, a significant advantage.

Great flexibility is provided in the application of RF plasma sourcepower to the torroidal plasma current. As discussed above, power istypically inductively coupled to the torroidal plasma current by anantenna. In many cases, the antenna predominantly is coupled to theexternal conduit or plenum by being close or next to it. For example, acoil antenna may extend alongside the conduit or plenum. However, inother cases the antenna is confined to the region enclosed between theconduit or plenum and the main reactor enclosure (e.g., the ceiling). Inthe latter case, the antenna may be considered to be “under” the conduitrather than alongside of it. Even greater flexibility is afford by caseshaving a magnetic core (or cores) extending through the enclosed region(between the conduit and the main chamber enclosure) and having anextension beyond the enclosed region, the antenna being wound around thecore's extension. In this case the antenna is inductively coupled viathe magnetic core and therefore need not be adjacent the torroidalplasma current in the conduit. In one such case, a closed magnetic coreis employed and the antenna is wrapped around the section of the corethat is furthest away from the torroidal plasma current or the conduit.Therefore, in effect, the antenna may be located almost anywhere, suchas a location entirely remote from the plasma chamber, by remotelycoupling it to the torroidal plasma current via a magnetic core.

Finally, plasma distribution over the surface of a very large diameterwafer or workpiece is uniform. This is accomplished in one case byshaping the torroidal plasma current as a broad plasma belt having awidth preferably exceeding that of the wafer. In another case,uniformity of plasma ion density across the wafer surface is achieved byproviding two or more mutually transverse or orthogonal torroidal plasmacurrents that intersect in the process region over the wafer. Thetorroidal plasma currents flow in directions mutually offset from oneanother by 360/n. Each of the torroidal plasma currents may be shaped asa broad belt of plasma to cover a very large diameter wafer. Each one ofthe torroidal plasma currents may be powered by a separate coil antennaaligned along the direction of the one torroidal plasma current. In onepreferred case, uniformity is enhanced by applying RF signals ofdifferent phases to the respective coil antennas so as to achieve arotating torroidal plasma current in the process region overlying thewafer. In this preferred case, the optimum structure is one in which thetorroidal plasma current flows in a circularly continuous plenumcommunicating with the main chamber portion through a circularlycontinuous annular opening in the ceiling or side wall. This latterfeature allows the entire torroidal plasma current to rotate azimuthallyin a continuous manner.

Controlling Radial Distribution of Plasma Ion Density:

FIG. 44 illustrates a plasma reactor similar to that illustrated in FIG.17A having a pair of orthogonal external reentrant tubes 150-1, 150B2.RF power is coupled into the tubes by respective annular magnetic cores1015-1, 1015-2 excited by respective RF-driven coils 170-1, 170-2, asdescribed above with reference to FIG. 17A. However, in FIG. 44 theexternal tubes 150-1, 150-2 are rectangular as in FIG. 24 rather thanbeing round in cross-sectional shape. Moreover, the horizontal sectionof the lower tube 150-1 is not flat but rather has a dip 4410 at itsmiddle. The dip 4410 permits the upper external tube 150-2 to nestcloser to the reactor ceiling 110. This feature shortens the path lengthin the upper tube 150-2, thereby reducing plasma losses in the uppertube 150-2. In fact, the shape of the dip 4410 may be selected to atleast nearly equalize the path length through the upper and lowerexternal tubes 150-1, 150-2. The reactor of FIG. 44, like the reactorsof FIGS. 2 and 26, has a gas distribution plate 210 on the ceiling 110(or forming the ceiling 110 itself) and overlying the wafer 120.

The dip 4410 is limited in that a vertical space remains between the topsurface of the ceiling 110 and a bottom corner 4422 formed on the lowertube 150-1 at the apex of the dip 4410. The vertical space accommodatesan electromagnet assembly 4430 that enhances plasma ion density over thecenter of the wafer 120. The electromagnet assembly 4430 includes anarrow elongate cylindrical pole piece 4440 formed of a magnetizablemetal such as iron or steel (for example) and a coil 4450 of insulatedconductive wire (e.g., copper wire) wrapped around the pole piece 4440.The cylindrical axis of the pole piece 4440 coincides with the axis ofsymmetry of the cylindrical chamber 100, so that the axis of the polepiece 4440 intersects the center of the wafer 120. The coil 4450 may bewrapped directly on the pole piece 4440 or, as illustrated in FIG. 45,may be wrapped around a mandrill 4460 encircling the pole piece 4440.FIG. 45 shows that the coil 4450 may be wrapped around a section 4440-1of the pole piece 4440 that extends above the ceiling 110. The lowersection 4440-2 of the pole piece 4440 that is inside the ceiling 110terminates within the gas manifold 220 of the gas distribution plate210.

For efficiency, it is desirable to place the source of theplasma-confining magnetic field as close to the plasma as practicalwithout disturbing gas flow within the gas distribution plate 210. Forthis purpose, the portion of the lower pole piece section 4440-2 that isinside the gas manifold 220 is a very narrow cylindrical end piece 4470that terminates the pole piece 4440. The end piece 4470 extends themagnetic field lines of the pole piece 4440 near the bottom of the gasdistribution plate to enhance the effect of the magnetic field on theplasma. The diameter of the end piece 4470 is sufficiently reduced sothat it does not appreciably interfere with gas flow within the gasmanifold 210. Moreover, such a reduced diameter brings the peak of theradial component of the magnetic field nearer the center axis.

FIG. 46 illustrates one case of the end piece 4470 having a taperedbottom 4475 terminated in a nipple 4477. FIG. 47 illustrates a case ofthe end piece 4470 in which the bottom 4476 is flat. FIG. 48 illustratesa case of the end piece 4470 in which the bottom 4478 is round.

In one implementation, pole piece 4440 has a diameter of about 3.5 cm(such that the diameter of the approximately 60 turn coil 4450 is about6 cm) and is about 12 cm long. The pole piece 4440 is extended about 2cm (to a total of about 14 cm) with a smaller diameter extension ofabout 1 cm diameter. The bottom of the extension region of the polepiece 4440 is about 1.5 cm from the top of the plasma region. Thematerial composition of pole piece 4440 is selected to have sufficientlyhigh permeability (e.g., μr>or=100) and high saturation flux density(e.g. Bsat>1000 gauss) to maximize the magnetic flux density in theregion below the pole piece 4440 with minimum magnetizing force andcurrent. Note that because the magnetic path is “open” with pole piece4440 (not closed within the pole piece), the effective permeability isreduced relative to the material permeability. Depending on thelength/diameter ratio of the pole piece 4440, the μr “effective” istypically reduced to on the order of 10.

An optional shield 4479 of magnetic material such as iron shields plasmain the pair of tubes 150-1, 150-2 from the D.C. magnetic field of theelectromagnet assembly 4430. The shield 4479 includes an overhead plate4479 a and a cylindrical skirt 4479 b.

In the case of the gas distribution plate 210 illustrated in FIG. 45, atop plate 4480 is divided into radially inner and outer sections 4480 a,4480 b, each having many small gas flow holes 4481 extending through it,the inner and outer sections having annular flanges 4482-1, 4482-2,4482-3, 4482-4, forming vertical walls supporting the bottom surface ofthe ceiling 210 and forming therewith inner and outer gas manifolds 4483a, 4483 b separated by a wall formed by the annular flanges 4482-2,4482-3. In one case, there is no wall between the inner and outer gasmanifolds, so as to avoid any discontinuity in gas distribution withinthe chamber that such a wall may cause. A gas mixing layer 4484 belowthe top plate 4480 diverts gas flow from a purely vertical flowdirection and thereby induces multi-directional (or turbulent) gas flowthat improves uniform mixing of gases of different molecular weights.Such diverting of the gas flow from a purely downward flow direction hasthe added benefit of suppressing high velocity gas flow effects, inwhich high velocity gas flow through gas distribution plate orificesdirectly over the wafer would form localized concentrations of processgas on the wafer surface that disrupt process uniformity. Suppression ofhigh velocity gas flow effects enhances uniformity.

The gas mixing layer 4484 may consist of metal or ceramic foam of thetype well-known in the art. Or, as shown in FIG. 49, the gas mixturelayer 4484 may consist of plural perforation plates 4484-1, 4484-2 eachhaving many small gas orifices drilled through it, the holes in oneperforation plate being offset from the holes in the other perforationplate. A bottom plate 4485 of the gas distribution plate 210 has manysub-millimeter gas injection holes 4486 (FIG. 50) drilled through itwith large counterbored holes 4487 at the top of the bottom plate 4485.In one example, the sub-millimeter holes were between 10 and 30 mils indiameter, the counterbored holes were about 0.06 inch in diameter andthe bottom plate 4485 had a thickness of about 0.4 inch. Inner and outergas feed lines 4490, 4492 through the ceiling 110 furnish gas to theinner and outer top plates 4480 a, 4480 b, so that gas flow in radiallyinner and outer zones of the chamber may be controlled independently asa way of adjusting process uniformity.

It is believed that the radial component of the D.C. magnetic fieldproduced by the electromagnet assembly 4430 affects the radialdistribution of plasma ion density, and that it is this radial componentof the magnetic field that can be exploited to enhance plasma iondensity near the center of the chamber. It is believed that suchenhancement of plasma ion density over the wafer center arises from theinteraction of the D.C. magnetic field radial component with the plasmasheath electric field at the wafer surface producing azimuthal plasmacurrents tending to confine plasma near the wafer center. In absence ofthe D.C. magnetic field, the phenomenon of a reduced plasma ion densityat the center of the chamber extends over a very small circular zoneconfined closely to the center of the wafer 120, because in general thereactor of FIG. 44 tends to have an exceptionally uniform plasma iondensity even in absence of a correcting magnetic field. Therefore,correction of the center-low plasma ion density distribution requires aD.C. magnetic field having a relatively large radial component very nearthe center of the chamber or wafer 120. The small diameter of themagnetic pole piece 4440 produces a magnetic field having a large radialcomponent very close to the center of the wafer 120 (or center of thechamber). In accordance with conventional practice, the center is theaxis of symmetry of the cylindrical chamber at which the radius is zero.FIG. 51 illustrates the distribution of the magnetic field in anelevational view of the processing region over the wafer 120 between thewafer 120 and the gas distribution plate 210. The vectors in FIG. 51 arenormalized vectors representing the direction of the magnetic field atvarious locations. FIG. 52 illustrates the magnetic flux density of theradial component of the magnetic field as a function of radial location,one curve representing the radial field flux density near the bottomsurface of the gas distribution plate 210 and the other curverepresenting the radial field flux density near the surface of the wafer120. The peak of the flux density of the radial magnetic field componentis very close to the center, namely at about a radius of only one inchboth at the ceiling and at the wafer. Thus, the radial component of themagnetic field is tightly concentrated near the very small diameterregion within which the plasma ion density tends to be lowest. Thus, thedistribution of the radial component of the D.C. magnetic field producedby the electromagnet assembly 4430 generally coincides with the regionof low plasma ion density near the center of the chamber.

As mentioned above, it is felt that the radial component of the D.C.magnetic field interacts with the vertically oriented electric field ofthe plasma sheath near the wafer center to produce an azimuthallydirected force that generally opposes radial travel of plasma. As aresult, plasma near the center of the wafer is confined to enhanceprocessing within that region.

A basic approach of using the electromagnet assembly 4430 in an etchreactor is to find a D.C. current flow in the coil that produces themost uniform etch rate radial distribution across the wafer surface,typically by enhancing plasma ion density at the center. This is thelikeliest approach in cases in which the wafer-to-ceiling gap isrelatively small (e.g., one inch), since such a small gap typicallyresults in a center-low etch rate distribution on the wafer. Forreactors having a larger gap (e.g., two inches or more), the etch ratedistribution may not be center low, so that a different D.C. current maybe needed. Of course, the electromagnet assembly 4430 is not confined toapplications requiring improved uniformity of plasma ion density acrossthe wafer surface. Some applications of the electromagnet assembly mayrequire an electromagnet coil current that renders the plasma iondensity less uniform. Such applications may involve, for example, casesin which a field oxide thin film layer to be etched has a non-uniformthickness distribution, so that uniform results can be obtained only byproviding non-uniform plasma ion density distribution that compensatesfor the non-uniform field oxide thickness distribution. In such a case,the D.C. current in the electromagnet assembly can be selected toprovide the requisite non-uniform plasma ion distribution.

As shown in FIG. 45, the plasma reactor may include a set of integratedrate monitors 4111 that can observe the etch rate distribution acrossthe wafer 120 during the etch process. Each monitor 4111 observes theinterference fringes in light reflected from the bottom of contact holeswhile the holes are being etched. The light can be from a laser or maybe the luminescence of the plasma. Such real time observation can makeit possible to determine changes in etch rate distribution across thewafer that can be instantly compensated by changing the D.C. currentapplied to the electromagnet assembly 4430.

FIG. 53 shows one way of independently controlling process gas flow tothe inner and outer gas feed lines 4490, 4492. In FIG. 53, one set ofgas flow controllers 5310, 5320, 5330 connected to the inner gas feedline 4490 furnish, respectively, argon, oxygen and a fluoro-carbon gas,such as C4F6, to the inner gas feed line 4490. Another set of gas flowcontrollers 5340, 5350, 5360 furnish, respectively, argon, oxygen and afluoro-carbon gas, such as C4F6, to the outer gas feed line 4492. FIG.54 shows another way of independently controlling process gas flow tothe inner and outer gas feed lines 4490, 4492. In FIG. 54, a single setof gas flow controllers 5410, 5420, 5430 furnishes process gases (e.g.,argon, oxygen and a fluoro-carbon gas) to a gas splitter 5440. The gassplitter 5440 has a pair of gas or mass flow controllers (MFC's) 5442,5444 connected, respectively, to the inner and outer gas feed lines4490, 4492. In addition, optionally another gas flow controller 5446supplies purge gas such as Argon or Neon to the outer gas feed line4492.

One problem in processing a large diameter wafer is that the torroidalor reentrant plasma current must spread out evenly over the wide surfaceof the wafer. The tubes 150 typically are less wide than the processarea. The need then is to broaden the plasma current to better cover awide process area as it exits a port 155 or 160. As related problem isthat the reactor of FIG. 44 (or any of the reactors of FIGS. 1-43) canexperience a problem of non-uniform plasma ion density and consequent“hot spot” or small region 5505 of very high plasma ion density near aport 155 or 160 of the reentrant tube 150, as shown in FIG. 55A.Referring to FIGS. 55A-56B, these problems are addressed by theintroduction of a plasma current flow splitter 5510 at the mouth of eachport (e.g., the port 155 as shown in FIG. 55A). The splitter 5510 tendsto force the plasma current to widen while at the same time reducingplasma ion density in the vicinity of the region 5505 where a hot spotmight otherwise form. The tube 150 can have a widened terminationsection 5520 at the port 155, the termination section 5520 having adiameter nearly twice as great as that of the remaining portion of thetube 150. The plasma current flow splitter 5510 of FIG. 55A istriangular in shape, with one apex facing the interior of the tube 150so as to force the plasma current flowing into the chamber 100 from thetube 150 to spread out so as to better fill the larger diameter of thetermination section 5520. This current-spreading result produced by thetriangular splitter 5510 tends to widen the plasma current and reducesor eliminates the “hot spot” in the region 5505.

The optimum shape of the splitter 5510 depends at least in part upon theseparation distance S between the centers of opposing ports 155, 160. Ifthe splitter is too long in the direction of plasma flow (i.e., thevertical direction in FIG. 55A), then current flow along the dividedpath tends to be unbalanced, with all current flowing along one side ofthe splitter 5510. On the other hand, if the splitter 5510 is too short,the two paths recombine before the plasma current appreciably widens.

For example, in a chamber for processing a 12-inch diameter wafer, theseparation distance S can be about 20.5 inches, with a tube width w of 5inches, a tube draft d of 1.75 inches and an expanded terminationsection width W of 8 inches. In this case, the juxtaposition of the port155 relative to the 12 inch wafer would be as shown in the plan view ofFIG. 56C. In this particular example, the height h of the splitter 5510should be about 2.5 inches, with the angle of the splitter's apex 5510 abeing about 75 degrees, as shown in FIG. 57. In addition, the length Lof the termination section 5520 should equal the height h of thesplitter 5510.

On the other hand, for a separation distance S of 16.5 inches, anoptimum splitter 5510′ is illustrated in FIG. 58. The angle of thesplitter apex in this case is preferably about 45 degrees, thetriangular portion being terminated in a rectangular portion having awidth of 1.2 inches and a length such that the splitter 5510′ has aheight h of 2.5 inches. The height and apex angle of the splitter 5510or 5510′ must be sufficient to reduce plasma density in the region 5505to prevent formation of a hot spot there. However, the height h must belimited in order to avoid depleting plasma ion density at the wafercenter.

FIGS. 59A and 59B illustrate splitters for solving the problem of plasmaion density non-uniformity near the entrance ports of a reentrant tube2654 in which plasma current flow through each port is in a horizontaldirection through the chamber side wall 105, as in the reactor of FIG.26. Each splitter 5910 has its apex 5910 a facing the port 2680.

FIGS. 60, 61 and 62 illustrate an implementation like that of FIG. 17A,except that the chamber side wall 105 is rectangular or square and thevertically facing ports 140-1, 140-2, 140-3 and 140-4 through theceiling 110 are located over respective corners 105 a, 105 b, etc. ofthe rectangular or square side wall 105. A floor 6020 in the plane ofthe wafer 120 faces each port and, together with the corner-formingsections of the rectangular side wall 105, forces incoming plasmacurrent to turn toward the processing region overlying the wafer 120. Inorder to reduce or eliminate a hot spot in plasma ion density in theregion 6030, a triangular plasma current flow splitter 6010 is placednear each respective corner 105 a, 105 b, etc., with its apex 6010 afacing that corner. In the implementation of FIG. 61, the splitter apex6010 a is rounded, but in other implementations it may be less roundedor actually may be a sharp edge. FIG. 63 illustrates a portion of thesame arrangement but in which the edge 6010 b of the splitter 6010facing the wafer 120 is located very close to the wafer 120 and isarcuately shaped to be congruent with the circular edge of the wafer120. While the splitter 6010 of FIG. 60 extends from the floor 6020 tothe ceiling 110, FIG. 64 illustrates that the height of the splitter6010 may be less, so as to allow some plasma current to pass over thesplitter 6010.

As will be discussed in greater detail below with respect to certainworking examples, the total path length traversed by the reentrantplasma current affects plasma ion density at the wafer surface. This isbecause shorter path length places a higher proportion of the plasmawithin the processing region overlying the wafer, reduces pathlength-dependent losses of plasma ions and reduces surface area lossesdue to plasma interaction with the reentrant tube surface. Therefore,the shorter length tubes (corresponding to a shorter port separationdistance S) are more efficient. On the other hand, a shorter separationdistance S affords less opportunity for plasma current flow separated atits center by the triangular splitter 5510 to reenter the center regionafter passing the splitter 5510 and avoid a low plasma ion density atthe wafer center. Thus, there would appear to be a tradeoff between thehigher efficiency of a smaller port separation distance S and the riskof depressing plasma ion density at the wafer center in the effort toavoid a plasma hot spot near each reentrant tube port.

This tradeoff is ameliorated or eliminated in the case of FIGS. 65A, 65Band 66, by using a triangular splitter 6510 that extends at least nearlyacross the entire width W of the termination section 5520 of the portand is shaped to force plasma current flow away from the inner edge 6610of the port and toward the outer edge 6620 of the port. This featureleaves the port separation distance S unchanged (so that it may be asshort as desired), but in effect lengthens the plasma current path fromthe apex 6510 a of the splitter to the center of the wafer 120. Thisaffords a greater opportunity for the plasma current flow split by thesplitter 6510 to rejoin at its center before reaching the wafer orcenter of the wafer. This feature better avoids depressing plasma iondensity at the wafer center while suppressing formation of plasma hotspots at the reentrant tube ports.

As illustrated in FIGS. 65A, 65B and 66, each splitter 6510 presents anisosceles triangular shape in elevation (FIG. 65B) and a rectangularshape from the top (FIG. 65A). The side view of FIG. 66 reveals thesloping back surface 6610 c that extends downwardly toward the outeredge 6620 of the port. It is the sloping back surface 6610 c that forcesthe plasma current toward the back edge 6620 thereby effectivelylengthening the path from the top of the apex 6510 a to the wafercenter, which is the desired feature as set forth above. The rectangularopening of the port 150 is narrowed in the radial direction (the shortdimension) by the sloped wall or sloping back surface 6610 b from about2″ at top to about ¾″ at the bottom. This pushes the inner port edgeabout 1-¼″ radially farther from the wafer (thus achieving the desiredincrease in effective port separation distance). In addition, the port150 has the full triangular splitter 6510 in the azimuthal direction(the long or 8″ wide dimension of the opening 150).

The plasma current splitter 5510 or 6510 may have coolant passagesextending within it with coolant ports coupled to similar ports in thereactor body to regulate the temperature of the splitter. For thispurpose, the plasma current splitter 5510 or 6510 is formed of metal,since it easily cooled and is readily machined to form internal coolantpassages. However, the splitter 5510 or 6510 may instead be formed ofanother material such as quartz, for example.

FIG. 67 illustrates another way of improving plasma uniformity in thetorroidal source reactor of FIG. 24 by introducing a set of four annularelectromagnets 6710, 6720, 6730, 6740 along the periphery of thereactor, the windings of each electromagnet being controlled by a magnetcurrent controller 6750. The electric currents in the fourelectromagnets may be driven in any one of three modes:

-   -   in a first mode, a sinusoidal mode, the coils are driven at the        same low frequency current in phase quadrature to produce a        magnetic field that rotates about the axis of symmetry of the        reactor at the low frequency of the source;    -   in a second mode, a configurable magnetic field mode, the four        electromagnets 6710, 6720, 6730, 6740 are grouped into opposing        pairs of adjacent electromagnets, and each pair is driven with a        different D.C. current to produce a magnetic field gradient        extending diagonally between the opposing pairs of adjacent        electromagnets, and this grouping is rotated so that the        magnetic field gradient is rotated to isotropically distribute        its effects over the wafer;    -   in a third mode, the four electromagnets are all driven with the        same D.C. current to produce a cusp-shaped magnetic field having        an axis of symmetry coinciding generally with the axis of        symmetry of the reactor chamber.

As shown in FIG. 1, a pumping annulus is formed between the cylindricalwafer support pedestal 115 and the cylindrical side wall 105, gasesbeing evacuated via the pumping annulus by the vacuum pump 135. Plasmacurrent flow between the opposing ports of each reentrant tube 150 canflow through this pumping annulus and thereby avoid flowing through theprocessing region between the wafer 120 and the gas distribution plate210. Such diversion of plasma current flow around the process region canoccur if the chamber pressure is relatively high and thewafer-to-ceiling gap is relatively small and/or the conductivity of theplasma is relatively low. To the extent this occurs, plasma ion densityin the process region is reduced. This problem is solved as shown inFIGS. 68 and 69 by the introduction of radial vanes 6910, 6920, 6930,6940 blocking azimuthal plasma current flow through the pumping annulus.In one implementation, the vanes 6910, 6920, 6930, 6940 extend up to butnot above the plane of the wafer 120, to allow insertion and removal ofthe wafer 120. However, in another implementation the vanes mayretractably extend above the plane of the wafer to better confine theplasma current flow within the processing region overlying the wafer120. This may be accomplished by enabling the wafer support pedestal 115to move up and down relative to the vanes, for example. In either case,the vanes 6910, 6920, 6930, 6940 prevent plasma current flow through thepumping annulus, and, if the vanes can be moved above the plane of thewafer 120, they also reduce plasma current flow through the upper regionoverlying the pumping annulus. By thus preventing diversion of plasmacurrent flow away from the processing region overlying the wafer, notonly is plasma ion density improved in that region but process stabilityis also improved.

As mentioned previously herein, the magnetic core used to couple RFpower to each reentrant tube 150 tends to crack or shatter at high RFpower levels. It is believed this problem arises because magnetic fluxis not distributed uniformly around the core. Generally, one windingaround the core has a high current at high RF power levels. This windingcan be, for example, a secondary winding that resonates the primarywinding connected to the RF generator. The secondary winding isgenerally confined to a narrow band around the core, magnetic flux andheating being very high within this band and much lower elsewhere in thecore. The magnetic core must have a suitable permeability (e.g., apermeability between about 10 and 200) to avoid self-resonance at highfrequencies. A good magnetic core tends to be a poor heat conductor (lowthermal conductivity) and be readily heated (high specific heat), and istherefore susceptible to localized heating. Since the heating islocalized near the high current secondary winding and since the coretends to be brittle, it cracks or shatters at high RF power levels(e.g., 5 kilowatts of continuous power).

This problem is solved in the manner illustrated in FIGS. 70 through 74by more uniformly distributing RF magnetic flux density around theannular core. FIG. 70 illustrates a typical one of the magnetic cores1015 of FIG. 17A. The core 1015 is formed of a high magneticpermeability material such as ferrite. The primary winding 170 consistsof about two turns of a thin copper band optionally connected through animpedance match device 175 to the RF generator 180. High current flowrequired for high magnetic flux in the core 1015 occurs in a resonantsecondary winding 7010 around the core 1015. Current flow in thesecondary winding 7010 is about an order of magnitude greater thancurrent flow in the primary winding. In order to uniformly distributemagnetic flux around the core 1015, the secondary winding 7010 isdivided into plural sections 7010 a, 7010 b, 7010 c, etc., that areevenly distributed around the annular core 1015. The secondary windingsections 7010 a, etc., are connected in parallel. Such parallelconnection is facilitated as illustrated in FIGS. 71A and 71B by a pairof circular copper buses 7110, 7120 extending around opposite sides ofthe magnetic core 1015. Opposing ends of each of the secondary windings7010 a, 7010 b, etc., are connected to opposite ones of the two copperbuses 7110, 7120. The copper buses 7110, 7120 are sufficiently thick toprovide an extremely high conductance and low inductance, so that theazimuthal location of any particular one of the secondary windingsections 7010 a, 7010 b, etc. makes little or no difference, so that allsecondary winding sections function as if they were equidistant from theprimary winding. In this way, magnetic coupling is uniformly distributedaround the entire core 1015.

Because of the uniform distribution of magnetic flux achieved by theforegoing features, the primary winding may be placed at any suitablelocation, typically near a selected one of the plural distributedsecondary winding sections 7110 a, 7110 b, 7110 c, etc. However, in oneimplementation, the primary winding is wrapped around or on a selectedone of the plural distributed secondary winding sections 7110 a, 7110 b,7110 c, etc.

FIG. 72 is a representation of the distributed parallel inductancesformed by the parallel secondary winding sections 7010 a, 7010 b, etc.,and FIG. 73 shows the circular topology of these distributedinductances. In order to provide resonance at the frequency of the RFgenerator 180, plural distributed capacitors 7130 are connected inparallel across the two copper buses 7110, 7120. The plural capacitors7030 are distributed azimuthally around the magnetic core 1015. Eachcapacitor 7030 in one implementation was about 100 picoFarads. Theequivalent circuit of the distributed inductances and capacitancesassociated with the secondary winding 7010 is illustrated in FIG. 24.

Referring to FIG. 71B, the secondary winding sections 7010 a, 7010 b,etc., can have the same number of turns. In the case of FIG. 71B, thereare six secondary winding sections 7010 a-7010 f, each section havingthree windings. The skilled worker can readily select the number ofsecondary winding sections, the number of windings in each section andthe capacitance of the distributed capacitors 7030 to achieve resonanceat the frequency of the RF generator 180. The copper band stock used toform the primary and secondary windings around the core 1015 can be, forexample, 0.5 inch wide and 0.020 inch thick copper stripping. The twocopper buses 7110, 7120 are very thick (e.g., from 0.125 inch to 0.25inch thick) and wide (e.g., 0.5 inch wide) so that they form extremelylow resistance, low inductance current paths. The core 1015 may consistof a pair of stacked 1 inch thick ferrite cores with a 10 inch outerdiameter and an 8 inch inner diameter. Preferably, the ferrite core 1015has a magnetic permeability □=40. The foregoing details are provided byway of example only, and any or all of the foregoing values may requiremodification for different applications (e.g., where, for example, thefrequency of the RF generator is modified).

We have found that the feature of distributed inductances illustrated inFIGS. 71A and 71B solves the problem of breakage of the magnetic coreexperienced at sustained high RF power levels (e.g., 5 kilowatts).

FIG. 75 illustrates the equivalent circuit formed by the core andwindings of FIGS. 71A and 71B. In addition to the primary and secondarywindings 170 and 7010 around the core 1015, FIG. 75 illustrates theequivalent inductive and capacitive load presented by the plasmainductively coupled to the core 1015. The case of FIGS. 70-75 is atransformer coupled circuit. The purpose of the secondary winding 7010is to provide high electric current flow around the magnetic core 1015for enhanced power coupling via the core. The secondary winding 7010achieves this by resonating at the frequency of the RF generator. Thus,the high current flow and power coupling via the magnetic core 1015occurs in the secondary winding 7010, so that virtually all the heatingof the core 1015 occurs at the secondary winding 7010. By thusdistributing the secondary winding 7010 around the entire circumferenceof the core 1015, this heating is similarly distributed around the coreto avoid localized heating and thereby prevent shattering the core athigh RF power levels.

The distributed winding feature of FIGS. 71A and 71B can be used toimplement other circuit topologies, such as the auto transformer circuitof FIG. 76. In the auto transformer circuit of FIG. 76, the winding 7010around the core 1015 is distributed (in the manner discussed above withreference to FIGS. 70-74) and has a tap 7610 connected through theimpedance match circuit 175 to the RF generator 180. The distributedcapacitors 7030 provide resonance (in the manner discussed above). As inFIG. 70, the core 7010 is wrapped around the reentrant tube 150 so thatpower is inductively coupled into the interior of the tube 150. Thecircuit topologies of FIGS. 75 and 76 are only two examples of thevarious topologies that can employ distributed windings around themagnetic core 1015.

In one implementation, the impedance match circuits 175 a, 175 bemployed frequency tuning in which the frequency of each RF generator180 a, 180 b is controlled in a feedback circuit in such a way as tominimize reflected power and maximize forward or delivered power. Insuch an implementation, the frequency tuning ranges of each of thegenerators 180 a, 180 b are exclusive, so that their frequencies alwaysdiffer, typically on the order of a 0.2 to 2 MHz difference. Moreover,their phase relationship is random. This frequency difference canimprove stability. For example, instabilities can arise if the samefrequency is used to excite plasma in both of the orthogonal tubes150-1, 150-2. Such instabilities can cause the plasma current to flowthrough only three of the four ports 155, 160, for example. Thisinstability may be related to the phase difference between the torroidalplasma currents in the tubes. One factor facilitating plasma stabilityis isolation between the two plasma currents of the pair of orthogonaltubes 150-1, 150-2. This isolation is provided mainly by the plasmasheaths of the two plasma currents. The D.C. break or gap 152 of each ofthe reentrant tubes 150-1, 150-2 also enhances plasma stability.

While the D.C. break or gap 152 in each of the orthogonal tubes isillustrated in FIG. 44 as being well-above the chamber ceiling 110, itmay in fact be very close to or adjacent the ceiling. Such anarrangement is employed in the implementation of FIG. 77, in which thecase of FIG. 55A is modified so that the termination section 5520electrically floats so that its potential follows oscillations of theplasma potential. This solves a problem that can be referred to as a“hollow cathode” effect near each of the ports 155, 160 that createsnon-uniform plasma distribution. This effect may be referred to as anelectron multiplication cavity effect. By permitting all of theconductive material near a port to follow the plasma potentialoscillations, the hollow cathode effects are reduced or substantiallyeliminated. This is achieved by electrically isolating the terminationsection 5520 from the grounded chamber body by locating a D.C. break orgap 152′ at the juncture between the reentrant tube termination section5520 and the top or external surface of the ceiling 110. (The gap 152′may be in addition to or in lieu of the gap 152 of FIG. 44.) The gap152′ is filled with an insulative annular ring 7710, and the terminationsection 5520 of FIG. 77 has a shoulder 7720 resting on the top of theinsulative ring 7710. Moreover, there is an annular vacuum gap 7730 ofabout 0.3 to 3 mm width between the ceiling 110 and the terminationsection 5520. In one implementation, the tube 150 and the terminationsection 5520 are integrally formed together as a single piece. Thetermination section 5520 is preferably formed of metal so that internalcoolant passages may be formed therein.

FIGS. 44-77 illustrate cases in which the uniformity control magnet isabove the processing region. FIG. 78 illustrates that the magnet pole4440 may be placed below the processing region, or under the wafersupport pedestal 115.

Working Examples:

An etch process was conducted on blanket oxide wafers at a chamberpressure of 40 mT, 4800 watts of 13.56 MHz RF bias power on the waferpedestal and 1800 Watts of RF source power applied to each reentranttube 150 at 11.5 MHz and 12.5 MHz, respectively. The magnetic fieldproduced by the electromagnet assembly 4430 was set at the followinglevels in successive steps: (a) zero, (b) 6 Gauss and (c) 18 Gauss(where the more easily measured axial magnetic field component at thewafer center was observed rather than the more relevant radialcomponent). The observed etch rate distribution on the wafer surface wasmeasured, respectively, as (a) center low with a standard deviation ofabout 2% at zero Gauss, (b) slightly center fast with a standarddeviation of about 1.2% at 6 Gauss, and (c) center fast with a standarddeviation of 1.4%. These examples demonstrate the ability to providenearly ideal compensation (step b) and the power to overcompensate (stepc).

To test the effective pressure range, the chamber pressure was increasedto 160 mT and the electromagnet's field was increased in three stepsfrom (a) zero Gauss, to (b) 28 Gauss and finally to (c) 35 Gauss (wherethe more easily measured axial magnetic field component at the wafercenter was observed rather than the more relevant radial component). Theobserved etch rate was, respectively, (a) center slow with a standarddeviation of about 2.4%, center fast with a standard deviation of about2.9% and center fast with a standard deviation of about 3.3%. Obviously,the step from zero to 28 Gauss resulted in overcompensation, so that asomewhat smaller magnetic field would have been ideal, while the entireexercise demonstrated the ability of the electromagnet assembly 4430 toeasily handle very high chamber pressure ranges. This test was severebecause at higher chamber pressures the etch rate distribution tends tobe more severely center low while, at the same time, the decreasedcollision distance or mean free path length of the higher chamberpressure makes it more difficult for a given magnetic field to effectplasma electrons or ions. This is because the magnetic field can have noeffect at all unless the corresponding Larmour radius of the plasmaelectrons or ions (determined by the strength of the magnetic field andthe mass of the electron or ion) does not exceed the plasma collisiondistance. As the collision distance decreases with increasing pressure,the magnetic field strength must be increased to reduce the Larmourradius proportionately. The foregoing examples demonstrate the power ofthe electromagnet assembly to generate a sufficiently strong magneticfield to meet the requirement of a small tarmour radius.

Another set of etch processes were carried out on oxide wafers patternedwith photoresist at 35 mT under similar conditions, and the currentapplied to the electromagnet assembly 4430 was increased in five stepsfrom (a) 0 amperes, (b) 5 amperes, (c) 6 amperes, (d) 7 amperes and (e)8 amperes. (In this test, a current of 5 amperes produces about 6 gaussmeasured axial magnetic field component at the wafer center.) At eachstep, the etch depths of high aspect ratio contact openings weremeasured at both the wafer center and the wafer periphery to testcenter-to-edge etch rate uniformity control. The measured center-to-edgeetch rate differences were, respectively, (a) 13.9% center low, (b) 3.3%center low, (c) 0.3% center low, (d) 2.6% center high and (e) 16.3%center high. From the foregoing, it is seen that the ideal electromagnetcurrent for best center-to-edge uniformity is readily ascertained and inthis case was about 6 amperes.

A set of etch processes were carried out on blanket oxide wafers to testthe efficacy of the dual zone gas distribution plate 210 of FIG. 44. Ina first step, the gas flow rates through the two zones were equal, in asecond step the inner zone had a gas flow rate four times that of theouter zone and in a third step the outer zone had a gas flow rate fourtimes that of the inner zone. In each of these steps, no current wasapplied to the electromagnet assembly 4430 so that the measurementstaken would reflect only the effect of the dual zone gas distributionplate 210. With gas flow rates of the two zones equal in the first step,the etch rate distribution was slightly center high with a standarddeviation of about 2.3%. With the inner zone gas flow rate at four timesthat of the outer zone, the etch rate distribution was center fast witha standard deviation of about 4%. With the outer zone gas flow rate atfour times that of the inner zone, the etch rate distribution was centerslow with a standard deviation of about 3.4%. This showed that the dualzone differential gas flow rate feature of the gas distribution plate210 can be used to make some correction to the etch rate distribution.However, the gas flow rate control directly affects neutral speciesdistribution only, since none of the incoming gas is (or should be)ionized. On the other hand, etch rate is directly affected by plasma iondistribution and is not as strongly affected by neutral distribution, atleast not directly. Therefore, the etch rate distribution controlafforded by the dual zone gas distribution plate, while exhibiting someeffect, is necessarily less effective than the magnetic confinement ofthe electromagnet assembly 4430 which directly affects plasma electronsand thus ions.

The dependency of the electromagnet assembly 4430 upon the reentranttorroidal plasma current was explored. First a series of etch processeswas carried out on blanket oxide wafers with no power applied to thetorroidal plasma source, the only power being 3 kiloWatts of RF biaspower applied to the wafer pedestal. The electromagnet coil current wasincreased in four steps of (a) zero amperes, (b) 4 amperes, (c) 6amperes and (d) 10 amperes. The etch rate distribution was observed inthe foregoing steps as (a) center high with a standard deviation of2.87%, (b) center high with a standard deviation of 3.27%, (c) centerhigh with a standard deviation of 2.93% and (d) center high with astandard deviation of about 4%. Thus, only a small improvement inuniformity was realized for a relatively high D.C. current applied tothe electromagnet assembly 4430. Next, a series of etch processes wascarried out under similar conditions, except that 1800 Watts was appliedto each of the orthogonal tubes 150-1, 150-2. The electromagnet coilcurrent was increased in six steps of (a) zero amperes, (b) 2 amperes,(c) 3 amperes, (d) 4 amperes, (e) 5 amperes and (f) 6 amperes. The etchrate distribution was, respectively, (a) center low with a standarddeviation of 1.2%, (b) center low with a standard deviation of 1.56%,(c) center high with a standard deviation of 1.73%, (d) center high witha standard deviation of 2.2%, (e) center high with a standard deviationof 2.85% and (f) center high with a standard deviation of 4.25%.Obviously the most uniform distribution lies somewhere between 2 and 3amperes where the transition from center low to center high was made.Far greater changes in plasma distribution were made using much smallercoil current with much smaller changes in coil current. Thus, thepresence of the reentrant torroidal plasma currents appears to enhancethe effects of the magnetic field of the electromagnet assembly 4430.Such enhancement may extend from the increase in bias power that ispossible when the torroidal plasma source is activated. In its absence,the plasma is less conductive and the plasma sheath is much thicker, sothat the bias RF power applied to the wafer pedestal must necessarily belimited. When the torroidal plasma source is activated (e.g., at 1800Watts for each of the two orthogonal tubes 150-1, 150-2) the plasma ismore conductive, the plasma sheath is thinner and more bias power can beapplied. As stated before herein, the effect of the D.C. magnetic fieldmay be dependent upon the interaction between the D.C. magnetic fieldand the electric field of the plasma sheath, which in turn depends uponthe RF bias power applied to the pedestal. Furthermore, the reentranttorroidal plasma currents may be attracted to the central plasma regiondue to the aforementioned postulated interaction between D.C. magneticfield and the electric field of the plasma sheath, further enhancing theplasma ion density in that region.

The effects of the port-to-port separation distance S of FIG. 55A wereexplored in another series of etch processes on blanket oxide wafers.The same etch process was carried out in reactors having separationdistances S of 16.5 inches and 20.5 inches respectively. The etch ratein the one with smaller separation distance was 31% greater than in theone with the greater separation distance (i.e., 6993 vs 5332Angstroms/minute) with 1800 Watts applied to each one of the orthogonaltubes 150-1, 150-2 with zero current applied to the electromagnetassembly 4300 in each reactor.

The effects of the port-to-port separation distance S of FIGS. 55-56were also explored in another series of etch processes on oxide waferspatterned with photoresist. With 3.7 amperes applied to theelectromagnet assembly 4300 having the smaller source (16.5 inch)separation distance S, the etch rate was 10450 Angstroms/minute vs 7858Angstroms/minute using the larger source (20.5 inch) separation distanceS. The effect of increasing power in the reactor having the greater(20.5 inches) separation distance S was explored. Specifically, the sameetch process was carried out in that reactor with source power appliedto each of the orthogonal tubes 150-1, 150-2 being 1800 Watts and thenat 2700 Watts. The etch rate increased proportionately very little,i.e., from 7858 Angstroms/minute to 8520 Angstroms/minute. Thus, theeffect of the port-to-port separation distance S on plasma ion densityand etch rate cannot readily be compensated by changing plasma sourcepower. This illustrates the importance of cases such as the case ofFIGS. 65A, 65B and 66 in which a relatively short port-to-portseparation distance S is accommodated while in effect lengthening thedistance over which the plasma current is permitted to equilibrate afterbeing split by the triangular splitters 5440.

The pole piece 4440 has been disclosed a being either a permanent magnetor the core of an electromagnet surrounded by a coil 4450. However, thepole piece 4440 may be eliminated, leaving only the coil 4450 as an aircoil inductor that produces a magnetic field having a similarorientation to that produced by the pole piece 4440. The air coilinductor 4450 may thus replace the pole piece 4440. Therefore, in moregeneral terms, what is required to produce the requisite radial magneticfield is an elongate pole-defining member which may be either the polepiece 4440 or an air coil inductor 4450 without the pole piece 4440 orthe combination of the two. The diameter of the pole-defining member isrelatively narrow to appropriately confine the peak of the radialmagnetic field.

Plasma Immersion Ion Implantation:

Referring to FIG. 79, a plasma immersion ion implantation reactor inaccordance with one aspect of the invention includes a vacuum chamber8010 having a ceiling 8015 supported on an annular side wall 8020. Awafer support pedestal 8025 supports a semiconductor (e.g., silicon)wafer or workpiece 8030. A vacuum pump 8035 is coupled to a pumpingannulus 8040 defined between the pedestal 8025 and the side wall 8020. Abutterfly valve 8037 regulates gas flow into the intake of the pump 8035and controls the chamber pressure. A gas supply 8045 furnishes processgas containing a dopant impurity into the chamber 8010 via a system ofgas injection ports that includes the injection port 8048 shown in thedrawing. For example, if the wafer 8030 is a crystalline silicon wafer aportion of which is to be implanted with a p-type conductivity dopantimpurity, then the gas supply 8045 may furnish BF₃ and/or B₂H₆ gas intothe chamber 8010, where Boron is the dopant impurity species. Generally,the dopant-containing gas is a chemical consisting of the dopantimpurity, such as boron (a p-type conductivity impurity in silicon) orphosphorus (an n-type conductivity impurity in silicon) and a volatilespecies such as fluorine and/or hydrogen. Thus, fluorides and/orhydrides of boron, phosphorous or other dopant species such as arsenic,antimony, etc., can be dopant gases. In a plasma containing a fluorideand/or hydride of a dopant gas such as BF₃, there is a distribution ofvarious ion species, such as BF₂+, BF+, B+, F+, F− and others (such asinert additives). All types of species may be accelerated across thesheath and may implant into the wafer surface. The dopant atoms (e.g.,boron or phosphorous atoms) typically dissociate from the volatilespecies atoms (e.g., fluorine or hydrogen atoms) upon impact with thewafer at sufficiently high energy. Although both the dopant ions andvolatile species ions are accelerated into the wafer surface, someportion of the volatile species atoms tend to leave the wafer during theannealing process that follows the ion implantation step, leaving thedopant atoms implanted in the wafer.

A plasma is generated from the dopant-containing gas within the chamber8010 by an inductive RF power applicator including an overhead coilantenna 8050 coupled to an RF plasma source power generator 8055 throughan impedance match circuit 8060. An RF bias voltage is applied to thewafer 8030 by an RF plasma bias power generator 8065 coupled to thewafer support pedestal 8025 through an impedance match circuit 8070. Aradially outer coil antenna 8052 may be driven independently by a secondRF plasma source power generator 8057 through an impedance match circuit8062.

The RF bias voltage on the wafer 8030 accelerates ions from the plasmaacross the plasma sheath and into the wafer surface, where they arelodged in generally interstitial sites in the wafer crystal structure.The ion energy, ion mass, ion flux density and total dose may besufficient to amorphize (damage) the structure of the wafer. The massand kinetic energy of the dopant (e.g., boron) ions at the wafer surfaceand the structure of the surface itself determine the depth of thedopant ions below the wafer surface. This is controlled by the magnitudeof the RF bias voltage applied to the wafer support pedestal 8025. Afterthe ion implantation process is carried out, the wafer is subjected toan anneal process that causes the implanted dopant atoms to move intosubstitutional atomic sites in the wafer crystal. The substrate surfacemay not be crystalline if it has been pre-amorphized prior to the plasmaimmersion ion implant process, or if the ion energy, ion mass, ion fluxdensity and total dose of plasma immersion ion implant process itself issufficient to amorphize the structure of the wafer. In such a case, theanneal process causes the amorphous (damaged) layer to recrystallizewith the incorporation and activation of implanted dopant. Theconductance of the implanted region of the semiconductor is determinedby the junction depth and the volume concentration of the activatedimplanted dopant species after the subsequent anneal process. If, forexample, a p-type conductivity dopant such as boron is implanted into asilicon crystal which has been previously doped with an n-type dopantimpurity, then a p-n junction is formed along the boundaries of thenewly implanted p-type conductivity region, the depth of the p-njunction being the activated implanted depth of the p-type dopantimpurities after anneal. The junction depth is determined by the biasvoltage on the wafer (and by the anneal process), which is controlled bythe power level of the RF plasma bias power generator 8065. The dopantconcentration in the implanted region is determined by the dopant ionflux (“dose”) at the wafer surface during implantation and the durationof the ion flux. The dopant ion flux is determined by the magnitude ofthe RF power radiated by the inductive RF power applicator 8050, whichis controlled by the RF plasma source power generator 8055. Thisarrangement enables independent control of the time of implant, theconductivity of the implanted region and the junction depth. Generally,the control parameters such as the power output levels of the bias powerRF generator 8065 and the source power RF generator 8055 are chosen tominimize the implant time while meeting the target values forconductivity and junction depth. For more direct control of ion energy,the bias generator may have “voltage” rather than “power” as its outputcontrol variable.

An advantage of the inductive RF plasma source power applicator 8050 isthat the ion flux (the dopant dose rate) can be increased by increasingthe power level of the RF source power generator 8055 without aconcomitant increase in plasma potential. The bias voltage level iscontrolled by the RF bias power generator at a preselected value(selected for the desired implant depth) while the inductive RF sourcepower is increased to increase the ion flux (the dopant dose rate)without significantly increasing the plasma potential. This featureminimizes contamination due to sputtering or etching of chambersurfaces. It further reduces the consumption of consumable componentswithin the chamber that wear out over time due to plasma sputtering.Since the plasma potential is not necessarily increased with ion flux,the minimum implant energy is not limited (increased), thereby allowingthe user to select a shallower junction depth than would otherwise havebeen possible. In contrast, it will be recalled that the microwave ECRplasma source was characterized by a relatively high minimum plasmapotential, which therefore limited the minimum implant energy andtherefore limited the minimum junction depth.

An advantage of applying an RF bias voltage to the wafer (instead of aD.C. bias voltage) is that it is far more efficient (and therefore moreproductive) for ion implantation, provided the RF bias frequency issuitably chosen. This is illustrated in FIGS. 80A, 80B and 80C. FIG. 80Aillustrates a one-millisecond D.C. pulse applied to the wafer inconventional practice, while FIG. 80B illustrates the resulting ionenergy at the wafer surface. The D.C. pulse voltage of FIG. 80A is nearthe target bias voltage at which ions become substitutional uponannealing at the desired implant junction depth. FIG. 80B shows how theion energy decays from the initial value corresponding to the voltage ofthe pulse of FIG. 80A, due to resistive-capacitive effects at the wafersurface. As a result, only about the first micro-second (or less) of theone-millisecond D.C. pulse of FIG. 80A is actually useful, because it isonly this micro-second portion of the pulse that produces ion energiescapable of implanting ions that become substitutional (during annealing)at the desired junction depth. The initial (one microsecond) period ofthe D.C. pulse may be referred to as the RC time. During the remainingportion of the D.C. pulse, ions fail to attain sufficient energy toreach the desired depth or to become substitutional upon annealing, andmay fail to penetrate the wafer surface so as to accumulate in adeposited film that resists further implantation. This problem cannot besolved by increasing the pulse voltage, since this would produce a largenumber of ions that would be implanted deeper than the desired junctiondepth. Thus, ions are implanted down to the desired junction depthduring only about a tenth of a percent of the time. This increases thetime required to reach the target implant density at the desiredjunction depth. The resulting spread in energy also reduces theabruptness of the junction. In contrast, each RF cycle in a 1millisecond burst of a 1 MHz RF bias voltage illustrated in FIG. 80C hasan RF cycle time not exceeding the so-called RC time of FIG. 80B. As aresult, resistive-capacitive effects encountered with a pulsed D.C. biasvoltage are generally avoided with an RF bias voltage of a sufficientfrequency. Therefore, ions are implanted down to the desired junctiondepth during a far greater percentage of the time of the 1 MHz RF biasvoltage of FIG. 80C. This reduces the amount of time required to reach atarget implant density at the desired junction depth. Thus, the use ofan RF bias voltage on the wafer results in far greater efficiency andproductivity than a D.C. pulse voltage, depending upon the choice of RFfrequency.

The frequency of the RF bias is chosen to satisfy the followingcriteria: The RF bias frequency must be sufficiently high to have anegligible voltage drop across the pedestal (cathode) dielectric layers)and minimize sensitivity to dielectric films on the backside or frontside of the wafer and minimize sensitivity to chamber wall surfaceconditions or deposition of plasma by-products. Moreover, the frequencymust be sufficiently high to have a cycle time not significantlyexceeding the initial period (e.g., one micro-second) beforeresistive-capacitive (RC) effects reduce ion energy more than 2% belowthe target energy, as discussed immediately above. Furthermore, the RFbias frequency must be sufficiently high to couple across insulatingcapacitances such as films on the wafer surface, dielectric layers onthe wafer support pedestal, coatings on the chamber walls, or depositedfilms on the chamber walls. (An advantage of RF coupling of the biasvoltage to the wafer is that such coupling does not rely upon ohmiccontact and is less affected by changes or variations in the surfaceconditions existing between the wafer and the support pedestal.)However, the RF bias frequency should be sufficiently low so as to notgenerate significant plasma sheath oscillations (leaving that task tothe plasma source power applicator). More importantly, the RF biasfrequency should be sufficiently low for the ions to respond to theoscillations of the electric field in the plasma sheath overlying thewafer surface. The considerations underlying this last requirement arenow discussed with reference to FIGS. 81A through 81D.

FIG. 81A illustrates the plasma ion saturation current at the wafersurface as a function of D.C. bias voltage applied to the wafer, thecurrent being greatest (skewed toward) the higher voltage region. FIG.81B illustrates the oscillation of the RF voltage of FIG. 80C. Theasymmetry of the ion saturation current illustrated in FIG. 80A causesthe ion energy distribution created by the RF bias voltage of FIG. 80Bto be skewed in like manner toward the higher energy region, asillustrated in FIG. 80C. The ion energy distribution is concentratedmost around an energy corresponding to the peak-to-peak voltage of theRF bias on the wafer. But this is true only if the RF bias frequency issufficiently low for ions to follow the oscillations of the electricfield in the plasma sheath. This frequency is generally a low frequencyaround 100 kHz to 3 MHz, but depends on sheath thickness andcharge-to-mass ratio of the ion. Sheath thickness is a function ofplasma electron density at the sheath edge and sheath voltage. Referringto FIG. 81D, as this frequency is increased from the low frequency(denoted F1 in FIG. 81D) to a medium frequency (denoted F2 in FIG. 81D)and finally to a high frequency such as 13 MHz (denoted F3 in FIG. 81D),the ability of the ions to follow the plasma sheath electric fieldoscillation is diminished, so that the energy distribution is narrower.At the HF frequency (F3) of FIG. 81D, the ions do not follow the sheathelectric field oscillations, and instead achieve an energy correspondingto the average voltage of the RF bias voltage, i.e., about half the RFbias peak-to-peak voltage. As a result, the ion energy is cut in half asthe RF bias frequency increases to an HF frequency (for a constant RFbias voltage). Furthermore, at the medium frequency, we have found thatthe plasma behavior is unstable in that it changes sporadically betweenthe low frequency behavior (at which the ions have an energycorresponding to the peak-to-peak RF bias voltage) and the highfrequency behavior (at which the ions have an energy corresponding toabout half the peak-to-peak RF bias voltage). Therefore, by maintainingthe RF bias frequency at a frequency that is sufficiently low(corresponding to the frequency F1 of FIG. 81D) for the ions to followthe plasma sheath electric field oscillations, the RF bias peak-to-peakvoltage required to meet a particular ion implant depth requirement isreduced by a factor of nearly two, relative to behavior at a mediumfrequency (F2) or a high frequency (F3). This is a significant advantagebecause such a reduction in the required RF bias voltage (e.g., by afactor of two) greatly reduces the risk of high voltage arcing in thewafer support pedestal and the risk of damaging thin film structures onthe wafer. This is particularly important because in at least aparticular plasma immersion ion implantation source described later inthis specification, ion energies match those obtained in a conventionalion beam implanter, provided the plasma RF bias voltage is twice theacceleration voltage of the conventional ion beam implanter. Thus, at ahigh frequency plasma RF bias voltage, where ion energies tend to behalf those obtained at low frequency, the required plasma RF biasvoltage is four times the acceleration voltage of the conventional ionbeam implanter for a given ion energy level. Therefore, it is importantin a plasma immersion ion implantation reactor to exploit the advantagesof a low frequency RF bias voltage, to avoid the necessity of excessiveRF bias voltages.

Good results are therefore attained by restricting the RF bias powerfrequency to a low frequency range between 10 kHz and 10 MHz. Betterresults are obtained by limiting the RF bias power frequency to anarrower range of 50 kHz to 5 MHz. The best results are obtained in theeven narrower bias power frequency range of 100 kHz to 3 MHz. We havefound optimum results at about 2 MHz plus or minus 5%.

Both the RF source power generator 8055 and the RF bias power generator8065 may apply continuous RF power to the inductive power applicator8050 and the wafer pedestal 8025 respectively. However, either or bothof the generators 8055, 8065 may be operated in burst modes controlledby a controller 8075. The controller 8075 may also control the generator8057 in a burst mode as well if the outer coil antenna 8052 is present.Operation in an implementation not including the outer coil antenna 8057will now be described. The RF signals produced by each of the generators8055, 8065 may be pulse modulated to produce continuous wave (CW) RFpower in bursts lasting, for example, one millisecond with a repetitionrate on the order of 0.5 kHz, for example. Either one or both of the RFpower generators 8055, 8065 may be operated in this manner. If both areoperated in such a burst mode simultaneously, then they may be operatedin a push-pull mode, or in an in-synchronism mode, or in a symmetricalmode or in a non-symmetrical mode, as will now be described.

A push-pull mode is illustrated in the contemporaneous time domainwaveforms of FIGS. 82A and 82B, illustrating the RF power waveforms ofthe respective RF generators 8055 and 8065, in which the bursts of RFenergy from the two generators 8055, 8065 occur during alternate timewindows. FIGS. 82A and 82B illustrate the RF power waveforms of thegenerators 8055 and 8065, respectively, or vice versa.

An in-synchronism mode is illustrated in the contemporaneous time domainwaveforms of FIGS. 82C and 82D, in which the bursts of RF energy fromthe two generators 8055, 8065 are simultaneous. They may not benecessarily in phase, however, particularly where the two generators8055, 8065 produce different RF frequencies. For example, the RF plasmasource power generator 8055 may have a frequency of about 13 MHz whilethe RF plasma bias power generator 8065 may have a frequency of about 2MHz. FIGS. 82C and 82D illustrate the RF power waveforms of thegenerators 8055 and 8065, respectively, or vice versa.

In the foregoing examples, the pulse widths and pulse repetition ratesof the two RF generators 8055, 8065 may be at least nearly the same.However, if they are different, then the temporal relationship betweenthe bursts of the two generators 8055, 8065 must be selected. In theexample of the contemporaneous time domain waveforms of FIGS. 82E and82F, one of the generators 8055, 8065 produces shorter RF burstsillustrated in FIG. 82F while the other produces longer RF burstsillustrated in FIG. 82E. In this example, the bursts of the twogenerators 8055, 8065 are symmetrically arranged, with the shorterbursts of FIG. 82F centered with respect to the corresponding longerbursts of FIG. 82E. FIGS. 82E and 82F illustrate the RF power waveformsof the generators 8055 and 8065, respectively, or vice versa.

In another example, illustrated in the contemporaneous time domainwaveforms of FIGS. 82G and 82H, the shorter bursts (FIG. 82H) are notcentered relative to the corresponding longer bursts (FIG. 82G), so thatthey are asymmetrically arranged. Specifically, in this example theshorter RF bursts of FIG. 82H coincide with the later portions ofcorresponding ones of the long bursts of FIG. 82G. Alternatively, asindicated in dashed line in FIG. 82H, the short RF bursts of FIG. 82Hmay instead coincide with the earlier portions of corresponding ones ofthe long RF bursts of FIG. 82G. FIGS. 82G and 82H illustrate the RFpower waveforms of the generators 8055 and 8065, respectively, or viceversa.

The inductive RF source power applicator 8050 of FIG. 79 tends toexhibit a rapid increase in dissociation of fluorine-containing speciesin the plasma as plasma source power (and ion flux) is increased,causing undue etching of semiconductor films on the wafer during theimplantation process. Such etching is undesirable. A plasma immersionion implantation reactor that tends to avoid this problem is illustratedin FIG. 83A. The plasma immersion ion implantation reactor of FIG. 83Ahas a capacitive source power applicator constituting a conductive(metal) or semiconducting ceiling 8015′ electrically insulated from thegrounded side wall 8020 by an insulating ring 8017. Alternatively, theceiling may be metal, conductive, or semiconducting and be coating by aninsulating, conducting or semiconducting layer. The RF plasma sourcepower generator 8055 drives the ceiling 8015′ through the impedancematch circuit 8060 in the manner of a capacitive plate. Plasma isgenerated by oscillations in the plasma sheath produced by the RF powercapacitively coupled from the ceiling 8015′. In order to enhance suchplasma generation, the frequency of the plasma RF source power generator8055 is relatively high, for example within the very high frequency(VHF) range or 30 MHz and above. The wafer pedestal 8025 may serve as acounter electrode to the ceiling 8015′. The ceiling 8015′ may serve as acounter electrode to the RF bias voltage applied to the wafer pedestal8025. Alternatively, the chamber wall may serve as a counter electrodeto either or both wafer bias and ceiling bias voltages. In oneimplementation, the dopant-containing gas is fed through the ceiling8015′ through plural gas injection orifices 8048′.

The capacitively coupled plasma ion immersion implantation reactor ofFIG. 83A enjoys the advantages of the inductively coupled reactor ofFIG. 79 in that both types of reactors permit the independent adjustmentof ion flux (by adjusting power level of the plasma source powergenerator 8055) and of the ion energy or implant depth (by adjusting thepower level of the plasma bias power generator 8065). In addition, whenplasma source power or ion flux is increased, the capacitively coupledplasma ion immersion reactor of FIG. 83A exhibits a smaller increase indissociation of fluorine-containing species in the gas fed from thedopant gas supply 8045 and a smaller increase in reaction by-productswhich would otherwise lead to excessive etch or deposition problems. Theadvantage is that ion flux may be increased more freely without causingan unacceptable level of etching or deposition during ion implantation.

The higher frequency RF power of the plasma source power generator 8055controls plasma density and therefore ion flux at the wafer surface, butdoes not greatly affect sheath voltage or ion energy. The lowerfrequency RF power of the bias power generator 8065 controls the sheathvoltage and therefore the ion implantation energy and (junction) depthand does not contribute greatly to ion generation or ion flux. Thehigher the frequency of the plasma source power generator, the lesssource power is wasted in heating ions in the plasma sheath, so thatmore of the power is used to generate plasma ions through oscillationsof the plasma sheath or by heating electrons in the bulk plasma. Thelower frequency of the RF bias power generator 8065 is less than 10 MHzwhile the higher frequency of the RF plasma source power generator 8055is greater than 10 MHz. More preferably, the lower frequency is lessthan 5 MHz while the higher frequency is greater than 15 MHz. Evenbetter results are obtained with the lower frequency being less than 3MHz and the higher frequency exceeding 30 MHz or even 50 MHz. In somecases the source power frequency may be as high as 160 MHz or over 200MHz. The greater the separation in frequency between the higher andlower frequencies of the source and bias power generators 8055, 8065,respectively, the more the plasma ion implant flux and the plasma ionimplant energy can be separately controlled by the two generators 8055,8065.

In the variation illustrated in FIG. 83B, the RF plasma source powergenerator 8055 is coupled to the wafer pedestal rather than beingcoupled to the ceiling 8015′. An advantage of this feature is that theceiling 8015′ is consumed (by plasma sputtering or etching) at a muchlower rate than in the reactor of FIG. 83A, resulting in less wear andless metallic contamination of the plasma. A disadvantage is thatisolation between the two RF generators 8055, 8065 from each other isinferior compared to the reactor of FIG. 83A, as they are both connectedto the same electrode, so that control of ion flux and ion energy is notas independent as in the reactor of FIG. 83A.

In either of the reactors of FIGS. 83A or 83B, the controller 8075 canoperate in the manner described above with reference to FIGS. 82Athrough 82H, in which the respective RF power waveforms applied to theceiling 8015′, and the pedestal 8025 are in a push-pull mode (FIGS. 82Aand B), or an in-synchronism mode (FIGS. 82C and D), or a symmetric mode(FIGS. 82E and 82F) or a non-symmetric mode (FIGS. 82G and H).

FIGS. 83A and 83B show that the RF source power generator 8055 can drivethe ceiling 8015′ (FIG. 83A) with the side wall 8020 and/or the wafersupport pedestal 8025 connected to the RF return terminal of thegenerator 8055, or, in the alternative, the RF source power generator8055 can drive the wafer support pedestal 8025 with the ceiling 8015′and/or the sidewall 8020 connected to the RF return terminal of thegenerator 8055. Thus, the RF source power generator is connected acrossthe wafer support pedestal 8025 and the sidewall 8020 or the ceiling8015′ (or both). The polarity of the connections to the source powergenerator 8055 may be reversed, so that it drives the side wall 8020and/or ceiling 8015′ with the pedestal 8025 being connected to the RFreturn terminal of the generator 8055.

As set forth above, the plasma immersion ion implantation inductivelycoupled reactor of FIG. 79 has distinct advantages, including (a) thecapability of a large ion flux/high plasma ion density, (b)independently controlled ion energy, and (c) low minimum ion energy(plasma potential). The plasma immersion ion implantation capacitivelycoupled reactor of FIG. 83A has the additional advantage of having morecontrollable dissociation of process gases and reactive byproducts asion flux is increased, than the inductively coupled reactor of FIG. 79.However, the capacitively coupled reactor of FIG. 83A has a higherminimum ion energy/plasma potential than the inductively coupled reactorof FIG. 79. Thus, these two types of reactors provide distinctadvantages, but neither provides all of the advantages.

A plasma immersion ion implantation reactor that provides all of theforegoing advantages, including low minimum ion energy and low processgas dissociation, is illustrated in FIG. 84. In FIG. 84, the inductivelyor capacitively coupled plasma sources of FIG. 79 or 83A are replaced bya torroidal plasma source of the type disclosed above in FIGS. 1-78. Inthe basic configuration of FIG. 84, the torroidal plasma source includesa reentrant hollow conduit 8150 over the ceiling 8015, corresponding tothe conduit 150 of FIG. 1. The conduit 8150 of FIG. 84 has one open end8150 a sealed around a first opening 8155 in the ceiling 8015 and anopposite open end 8150 b sealed around a second opening 8160 in theceiling 8015. The two openings or ports 8155, 8160 are located in theceiling over opposite sides of the wafer support pedestal 8025. WhileFIG. 84 illustrates the openings 8155, 8160 being in the ceiling, theopenings could instead be in the base or floor of the chamber, as inFIG. 12, or in the side wall of the chamber, as in FIG. 26, so that theconduit 8150 may pass over or under the chamber. RF plasma source poweris coupled from the RF generator 8055 through the optional impedancematch circuit 8060 to the reentrant conduit by an RF plasma source powerapplicator 8110. Various types of source power applicators for areentrant hollow conduit are disclosed in FIGS. 1-78, any one of whichmay be employed in the plasma immersion ion implantation reactor of FIG.84. In the implementation illustrated in FIG. 84, the RF plasma sourcepower applicator 8110 is similar to that illustrated in FIG. 13, inwhich a magnetically permeable core 8115 having a torus shape surroundsan annular portion of the conduit 8150. The RF generator 8055 is coupledthrough the optional impedance match circuit to a conductive winding8120 around the magnetic core 8115. An optional tuning capacitor 8122may be connected across the winding 8120. The RF generator 8055 may befrequency-tuned to maintain an impedance match, so that the impedancematch circuit 8060 may not be necessary.

The reactor chamber includes the process region 8140 between the wafersupport pedestal 8025 and the ceiling 8015. The gas supply 8045furnishes dopant gases into the reactor chamber 8140 through gasinjection orifices 8048 in the ceiling 8015. Plasma circulates(oscillates) through the reentrant conduit 8150 and across the processregion 8140 in response to the RF source power coupled by the sourcepower applicator 8110. As in the reactor of FIG. 13, the reentrantconduit 8150 is formed of a conductive material and has a narrow gap orannular break 8152 filled with an insulator 8154. The dopant gasesfurnished by the gas supply 8045 contain a species that is either adonor (N-type) or acceptor (P-type) impurity when substituted into thesemiconductor crystal structure of the wafer 8030. For example, if thewafer is a silicon crystal, then an N-type dopant impurity may bearsenic or phosphorous, for example, while a P-type dopant impurity maybe boron, for example. The dopant gas furnished by the gas supply 8045is a chemical combination of the dopant impurity with an at-leastpartially volatile species, such as fluorine for example. For example,if a P-type conductivity region is to be formed by ion implantation,then the dopant gas may be a combination of boron and fluorine, such asBF₃, for example. Or, for example, the dopant gas be a hydride, such asB₂H₆. Phosphorous doping may be accomplished using a fluoride such asPF₃ or PF₅ or a hydride such as PH₃. Arsenic doping may be accomplishedusing a fluoride such as AsF₅ or a hydride such as AsH₃.

The RF bias power generator provides an RF bias voltage, with the RFbias frequency selected as described above with reference to FIG. 81D.Good results are attained by restricting the RF bias power frequency toa low frequency range between 10 kHz and 10 MHz. Better results areobtained by limiting the RF bias power frequency to a narrower range of50 kHz to 5 MHz. The best results are obtained in the even narrower biaspower frequency range of 100 kHz to 3 MHz. We have found optimum resultsat about 2 MHz plus or minus 5%.

In the reactor of FIG. 84, both the RF source power generator 8055 andthe RF bias power generator 8065 may apply continuous RF power to theinductive power applicator 8110 and the wafer pedestal 8025respectively. However, either or both of the generators 8055, 8065 maybe operated in burst modes controlled by a controller 8075. The RFsignals produced by each of the generators 8055, 8065 may be pulsemodulated to produce continuous wave (CW) RF power in bursts lasting,for example, one millisecond with a repetition rate on the order of 0.5kHz, for example. Either one or both of the RF power generators 8055,8065 may be operated in this manner. If both are operated in such aburst mode simultaneously, then they may be operated in a push-pullmode, or in an in-synchronism mode, or in a symmetrical mode or in anon-symmetrical mode, as will now be described for the reactor of FIG.84.

A push-pull mode is illustrated in the contemporaneous time domainwaveforms of FIGS. 82A and 82B, illustrating the RF power waveforms ofthe respective RF generators 8055 and 8065, in which the bursts of RFenergy from the two generators 8055, 8065 occur during alternate timewindows. FIGS. 82A and 82B illustrate the RF power waveforms of thegenerators 8055 and 8065, respectively, or vice versa.

An in-synchronism mode is illustrated in the contemporaneous time domainwaveforms of FIGS. 82C and 82D, in which the bursts of RF energy fromthe two generators 8055, 8065 are simultaneous. They may not benecessarily in phase, however, particularly where the two generators8055, 8065 produce different RF frequencies. For example, the RF plasmasource power generator 8055 may have a frequency of about 13 MHz whilethe RF plasma bias power generator 8065 may have a frequency of about 2MHz. FIGS. 82C and 82D illustrate the RF power waveforms of thegenerators 8055 and 8065, respectively, or vice versa.

In the foregoing examples, the pulse widths and pulse repetition ratesof the two RF generators 8055, 8065 may be at least nearly the same.However, if they are different, then the temporal relationship betweenthe bursts of the two generators 8055, 8065 must be selected. In theexample of the contemporaneous time domain waveforms of FIGS. 82E and82F, one of the generators 8055, 8065 produces shorter RF burstsillustrated in FIG. 82F while the other produces longer RF burstsillustrated in FIG. 82E. In this example, the bursts of the twogenerators 8055, 8065 are symmetrically arranged, with the shorterbursts of FIG. 82F centered with respect to the corresponding longerbursts of FIG. 82E. FIGS. 82E and 82F illustrate the RF power waveformsof the generators 8055 and 8065, respectively, or vice versa.

In another example, illustrated in the contemporaneous time domainwaveforms of FIGS. 82G and 82H, the shorter bursts (FIG. 82H) are notcentered relative to the corresponding longer bursts (FIG. 82G), so thatthey are asymmetrically arranged. Specifically, in this example theshorter RF bursts of FIG. 82H coincide with the later portions ofcorresponding ones of the long bursts of FIG. 82G. Alternatively, asindicated in dashed line in FIG. 82H, the short RF bursts of FIG. 82Hmay instead coincide with the earlier portions of corresponding ones ofthe long RF bursts of FIG. 82G. FIGS. 82G and 82H illustrate the RFpower waveforms of the generators 8055 and 8065, respectively, or viceversa.

The torroidal plasma immersion ion implantation reactor of FIG. 84 canbe operated with a pulsed D.C. bias voltage instead of an RF biasvoltage. In this case, the bias power generator 8065 would be D.C.source rather than an RF source. Thus, in the different operationalmodes of FIGS. 82A through 82H discussed above, the pulsed RF biasvoltage may be replaced by a pulsed D.C. bias voltage of the same pulsewidth, with only the source power generator 8055 producing an RF powerburst.

FIG. 85 illustrates a modification of the plasma immersion ionimplantation reactor of FIG. 84 having a second reentrant conduit 8151crossing the first reentrant conduit 8150, in a manner similar to thereactor of FIG. 44. Plasma power is coupled to the second conduit 8151from a second RF plasma source power generator 8056 through a secondoptional match circuit 8061 to a second source power applicator 8111that includes a second magnetically permeable core 8116 and a secondcore winding 8121 driven by the second RF source power generator 8056.Process gas from the gas supply 8045 may be introduced into the chamberby a gas distribution plate or showerhead incorporated in the ceiling8015 (as in the gas distribution plate 210 of FIG. 44). However, theplasma immersion ion implantation reactor of FIG. 85 is greatlysimplified by using a small number of process gas injectors 8048 in theceiling 8015 or in the side wall 8020 or elsewhere, such as in the baseof the chamber (not shown) coupled to the dopant gas supply, rather thana showerhead. Moreover, the gap between the ceiling 8015 and the waferpedestal 8025 may be relatively large (e.g., two to six inches) and agas distribution plate eliminated in favor of discrete gas injectors ordiffuser 8048 in the ceiling 8015 or gas injectors or diffusers 8049 inthe side wall 8020 because there is no need to generate plasma close tothe wafer surface. The gas injectors or diffusers 8049 may be joined ina ring 8049 on the side wall 8020. Generally, the higher the maximumimplant depth and ion energy requirement, the greater the gap betweenceiling and wafer that is required. For example, for a peak-to-peak RFbias voltage of 10 kV, a gap of 4 inches is preferable over a 2 inch gapfor best plasma uniformity across a wide range of gas species and plasmaelectron densities. The term diffuser is employed in the conventionalsense as referring to a type of gas distribution device having a wideangle of gas flow distribution emanating from the device.

FIG. 86 is a plan view of the interior surface of the ceiling 8015,showing one arrangement of the gas injection orifices 8048, in whichthere is one central orifice 8048-1 in the center of the ceiling 8015and four radially outer orifices 8048-2 through 8048-5 uniformly spacedat an outer radius. FIG. 87 illustrates how the dopant gas supply 8045may be implemented as a gas distribution panel. The gas distributionpanel or supply 8045 of FIG. 87 has separate gas reservoirs 8210-1through 8210-11 containing different dopant-containing gases includingfluorides of boron, hydrides of boron, fluorides of phosphorous andhydrides of phosphorous. In addition, there are gas reservoirs for othergases used in co-implantation (hydrogen and helium), materialenhancement (nitrogen), surface passivation or co-implantation(fluorides of silicon or germanium or carbon). In addition, the centerorifice 8048-1 may be coupled to a reservoir oxygen gas, for use inphotoresist removal and/or chamber cleaning. A control panel 8220includes valves 8222 controlling gas flow from the respective reservoirs8210 to the gas injection orifices. Preferably, the gases are mixed ator near the orifices, although a gas manifold 8230 may be provided todistribute the selected gases among the outer gas injection orifices8048-2 through 8048-5. Alternatively, process gas may be injected at oneor more locations in the sidewall 8020, using the nozzles 8049 of FIG.85 or diffusers. FIG. 85 shows gas injectors 8049 located around thechamber sidewalls 8020 which inject gas radially inward. Gas may beinjected parallel to the ceiling and/or wafer, or may be injected withsome component toward ceiling and/or wafer. For some applications, it isadvantageous to utilize multiple separate gas plenums, each with its ownnozzle array. This can permit the use of chemistries which should not becombined except under vacuum, or may permit having several gas zones forneutral uniformity tuning. For this purpose, referring again to FIG. 85,a first ring 8049 a joining a first set of side wall injectors 8049 cserves as a first plenum, while a second ring 8049 b joining a secondseparate set of side wall injectors 8049 d serves as a second plenum.The two rings or plenums 8049 a, 8049 b are supplied by separaterespective sets of valves 8222 of the gas panel of FIG. 87

FIG. 88 illustrates a modification of the plasma immersion ionimplantation reactor of FIG. 85 in which a central electromagnetassembly 8430 is mounted over the center of the ceiling 8015. Like theelectromagnet assembly 4430 of FIG. 44, the electromagnet assembly 8430of FIG. 88 controls plasma ion density uniformity and includes a narrowelongate cylindrical pole piece 8440 formed of a magnetizable materialsuch as iron or steel and a coil 8450 of insulated conductive wirewrapped around the pole piece 8440. A magnetic current controller 8442supplies an electrical current to the coil 8450. The controller 8442controls the current through the coil 8450 so as to optimize uniformityof plasma ion density (ion flux) across the wafer surface.

FIGS. 89A and 89B are side and top views, respectively, illustrating afurther modification incorporating a radially outer electromagnetassembly 8460. The outer electromagnet assembly 8460 is in the shape ofa torus and overlies an annular outer region of the ceiling 8015 nearthe circumferential edge of the ceiling 8015 and adjacent the portspairs 150, 160 of the conduits 8150, 8151. Referring to thecross-sectional view of FIG. 90A, the outer electromagnet assembly 8460includes a coil 8462 consisting of plural windings of a single conductorconnected to the current controller 8442. In order to concentrate themagnetic field of the outer electromagnet assembly 8460 within theprocess region 8140, an overlying magnetic cover 8464 surrounding thesides and top of the coil 8462 but not the bottom of the coil 8462. Themagnetic cover 8464 permits the magnetic field of the coil 8462 toextend downwardly below the ceiling into the process region 8140.Uniformity of the ion density and radial plasma flux distribution at thewafer surface is optimized by independently adjusting the currents inthe inner and outer electromagnet assemblies 8430, 8460.

In order to avoid forming regions of very high plasma ion concentrationnear the ports 150, 160 of the two conduits 8150, 8151, individualplates 8466 of magnetically permeable material (e.g., iron or steel) areplaced under the outer electromagnet assembly 8460 adjacent respectiveones of the ports 150, 160. The circumferential extent of each plate8466 is approximately equal to the width of each individual port 150,160. FIGS. 90A, 90B and 90C are cross-sectional views taken along lines90—90 of FIG. 89B. The distance between the plate 8466 and the bottomedge of the magnetic cover 8464 may be adjusted to control the amount ofmagnetic field coupled into portion of the process region near eachindividual one of the ports 150, 160. In FIG. 90A, the plate 8466 is incontact with the bottom edges of the cover 8464, so that the magneticfield near the corresponding port (150, 160) is almost completelyconfined within the enclosure defined by the cover 8464 and the plate8466. In FIG. 90B, the plate 8466 is slightly displaced from the bottomedge of the cover 8464, creating a small gap therebetween that allows asmall magnetic field to enter the process region 8140 near thecorresponding port (150, 160). In FIG. 90C, there is a large gap betweenthe plate 8466 and the cover 8464, permitting a larger magnetic field toexist in the process region near the corresponding port (150, 160).

FIG. 91 illustrates how the RF plasma bias power generator 8065 may becoupled to the wafer support pedestal 8025. An inductor 8510 and avariable capacitor 8520 are connected in parallel between one side of aseries capacitor 8530 and ground, the other side of the series capacitor8530 being connected to the wafer support pedestal 8025. The output ofthe bias power generator 8065 is connected to a tap 8560 of the inductor8510. The position of the tap 8560 and the capacitance of the variablecapacitor 8520 are selected to provide an impedance match between thebias power generator 8065 and the plasma load at the wafer pedestal8065. The variable capacitor 8520 may be controlled by a systemcontroller 8525 to optimize matching. In this case, the circuitincluding the parallel inductor and capacitor 8510, 8520 serves as animpedance match circuit. In order to follow variations in the plasmaload impedance during processing, frequency tuning of the bias powergenerator 8065 may be employed, although this may not be necessary. Theposition of the tap 8560 may be selectable either manually or by thesystem controller 8525 to optimize matching. Alternatively, a capacitor(not shown) may be connected between the tap position and ground orbetween RF bias generator and tap point as an alternative matchingcircuit topology. This optional capacitor may be controlled by thesystem controller 8525 to optimize matching.

One problem in selecting the bias voltage level is that large ion energycan be reached only with a high bias voltage level, which typicallyrequires high power. High power contributes to the plasma flux (iondensity or dose rate), and can cause too high a dose rate, making itdifficult to control the conductivity of the implanted region. One wayof controlling the dose rate at such a high power is to pulse the RFbias power. However, controlling the pulse rate and pulse width ofrepetitive pulses so as to achieve the required dose rate andconductivity is difficult. Part of the problem is that ion implantationat the desired junction depth is achieved only after the bias voltagehas risen sufficiently (at the beginning of a pulse or RF burst) toreach a threshold voltage corresponding to the desired junction depthand ion energy. The solution to this problem is to avoid repetitivepulsing of the bias power, and instead use a single pulse of sufficientduration to complete ion implantation at the desired junction depth andconductivity in the implanted region. This is illustrated in the timedomain waveform of FIG. 92. A timer can be employed to guarantee thatthe RF burst or pulse lasts the required duration (Ttimer). However, thetimer must not begin until the sheath voltage has reached the thresholdvoltage (Vthreshold) at which ion implantation occurs at the requireddepth. Thus, FIG. 92 shows that the sheath voltage grows at thebeginning of bias power RF burst (Ton) until it reaches Vthreshold afterseveral cycles. At that point, the timer begins, and ends the RF burstat the expiration of Ttimer, i.e., at Toff. The problem, therefore, ishow to ascertain the time at which the sheath voltage reachesVthreshold, i.e., when to begin Ttimer.

Another problem is how to ascertain the requisite power level of thebias power generator 8065 at which Vthreshold is produced across thesheath.

FIG. 93 illustrates a control circuit for determining the bias generatorpower level that produces the desired sheath voltage and for determiningwhen the target sheath voltage has been reached for beginning the RFburst timer. In the following description, the target bias voltagecorresponding to a desired junction depth, has already been determined.In addition, the threshold voltage for implantation has also beendetermined, and the threshold voltage may be synonymous with the targetbias voltage. Finally, the duration time for applying RF bias power atthe target bias voltage has already been determined. The RF bias powergenerator 8065 is controlled by a timer 8670 that begins countingsometime after the beginning of an RF burst and times out after apredetermined duration. A threshold comparator 8672 compares thepeak-to-peak voltage as detected at the wafer pedestal 8025 by a peakdetector 8674 with the desired threshold voltage 8676. The timer 8670 isenabled only when it receives an affirmative signal from an opticaldetector 8678 indicating that plasma is ignited within the reactorchamber. If the optical detector 8678 sends an affirmative signal, thenthe timer 8670 begins counting as soon as the comparator 8672 determinesthat the peak-to-peak bias voltage has reached the desired threshold.When the timer 8670 times out (after the predetermined duration), itturns off the output of the bias power generator, thus terminating thecurrent burst of RF bias power. The timer 8670 and the thresholdcomparator 8672 constitute a timer control loop 8680.

The power level of the bias power generator 8065 is controlled by avoltage control loop 8682. A process controller 8684 (or the processdesigner) determines the desired or “target” bias peak-to-peak voltage.This may be synonymous with the threshold voltage of 8676. A subtractor8686 computes an error value as the difference between the actual peakbias voltage measured by the detector 8674 and the target bias voltage.A proportional integral conditioner 8688 multiplies this error value bya constant of proportionality, k, and integrates the error value withprior samples. The result is an estimated correction to the power levelof the bias power generator 8065 that will bring the measured biasvoltage closer to the target bias voltage. This estimate is superimposedon the current power level, and the result is an estimated power levelcommand that is applied to the power set input of the bias powergenerator 8065. This estimate is only valid while plasma is ignited(i.e., during an RF burst). For times between RF bursts, the bias powerlevel is controlled in accordance with a look-up table 8690 thatcorrelates target peak-to-peak bias voltages with estimated bias powerlevels. The look-up table receives the target bias voltage from theprocess controller 8684 and in response outputs an estimated bias powerlevel. A pair of switches 8694, 8696 are enabled in complementaryfashion by the output of the plasma ignition optical detector 8678.Thus, the switch 8694 receives the output of the sensor 8678 while theswitch 8696 receives the inverted output of the sensor 8678. Thus,during an RF burst, when plasma is ignited in the chamber, the output ofthe proportional integral conditioner 8688 is applied to the power setinput of the bias generator 8065 via the switch 8694. Between RF bursts,or when no plasma is ignited in the chamber, the output of the look-uptable 8690 is applied via the switch 8696 to the power set input of thebias power generator 8065. The output of the look up table 8690 may beconsidered as a gross estimate that serves to initialize the RF biaspower level at the beginning of each RF burst, while the output of theintegral proportional conditioner is a more accurate estimate based uponactual measurement that serves to correct the bias power level duringthe RF burst.

One problem in the plasma immersion ion implantation reactor of FIG. 89Ais that most ion implantation processes must be carried out with precisefine control over chamber pressure. This requires a relatively gradualchange in chamber pressure over a given rotation of the control valve8037 from its closed position. On the other hand, some processes,including chamber cleaning, require a very high gas flow rate (e.g., ofcleaning gases) and a concomitantly high evacuation rate by the pump8035. This requires that the vacuum control valve 8037 have a largearea. The problem is that with such a large area, a vacuum control valvedoes not provide the gradual change in pressure for a given rotationfrom its closed position that is necessary for fine control of chamberpressure during ion implantation. In fact, with a large area opening andflap, the change in chamber pressure is very rapid as the flap isrotated from its closed position, so that fine control of pressurewithin a very low pressure range, where the flap must be nearly closed,is very difficult. This problem is solved with the vacuum control valveof FIGS. 94, 95 and 96. The valve includes a flat housing 9410 having acircular opening 9412 through it. A rotatable flap 9420 having a diskshape is supported within the circular opening 9412 by a hinge 9422attached to the housing 9410. In its closed position, the flap 9420 isco-planar with the flat housing 9410. In order to prevent leakage ofplasma through the valve, the gap G between the rotatable flap 9420 andthe housing 9410 is narrow while the thickness T of the flap 9420 andhousing 9410 is large, much greater than the gap G. For example, theratio of the thickness T to the gap G is about 10:1. This featureprovides the advantage of frictionless operation. In order to providegradual control of chamber pressure at a very low pressure range (i.e.,when the flap 9420 is near its closed position), conically-shapedopenings 9430 are provided in the interior surface 9440 of the housing9410 defining the edge of the opening 9412. Some of the openings 9430have different axial locations (along the axis of the opening 9412) thanothers of the openings 9430. In its closed position, the flap 9420permits virtually zero gas leakage, because the openings 9430 are notexposed. As the flap 9420 begins to rotate from its closed position(i.e., in which the flap 9420 is co-planar with the housing 9410), smallportions of at least some of the openings 9430 begin to be exposed, andtherefore allow a small amount of gas flow through the valve. As theflap 9420 continues to rotate, it exposes larger portions of theopenings 9430. Moreover, it begins to expose others of the openings 9430not exposed during the earlier phase of its rotation due to thedifferent axial locations of different sets of the openings 9430, sothat the gas flows through more of the openings 9430 in proportion tothe rotation of the flap 9420. Thus, rotation of the flap 9430 from itsfully closed (co-planar) position causes a continuous but relativelygradual increase in gas flow through the openings 9430 until the bottomedge 9420 a of the flap 9420 reaches the top surface 9410 a of thehousing 9410. At this point, all of the openings 9430 are completelyexposed so that gas flow through the openings 9430 is maximum and cannotincrease further. Thus, a continuous gradual increase in gas flow isachieved (and therefore one that is readily controlled with a great dealof accuracy) as the flap 9420 rotates from its fully closed position tothe point at which the flap bottom edge 9420 a is aligned with thehousing top surface 9410 a. Within this range of flap rotationalposition, fine gradual adjustment of a small total chamber pressure isprovided. Further rotation of the flap 9420 creates an annular gapbetween the periphery of the flap 9420 and the periphery of the largecircular opening 9412, through which gas flow increases as the flap 9420continues to rotate.

The plural openings 9430 in the opening interior surface 9440 aresemi-circular openings that are tapered so as to increase in diametertoward the top housing surface 9410 a. The tapered semi-circularopenings 9430 thus define semi-conical shapes. However, other suitableshapes may be employed, such as semi-cylindrical, for example. However,one advantage of the semi-conical shape is that the rate of increase ofgas flow with rotational flap position may be enhanced as the rotationprogresses so that the rate continues to increase in a fairly smoothmanner after the transition point at which the flap bottom edge 9420 apasses the housing top surface 9410 a.

Depending upon the desired junction depth, the RF bias voltage appliedto the wafer support pedestal 8025 may be relatively small (e.g., 500volts) for a shallow junction or relatively large (e.g., 5,000 volts)for a deep junction. Some applications may require an RF bias voltage ofover 10,000 volts. Such large voltages can cause arcing within the wafersupport pedestal 8025. Such arcing distorts process conditions in thereactor. In order to enable the wafer support pedestal 8025 to withstandbias voltages as high a 10,000 volts, for example, without arcing, voidswithin the wafer support pedestal 8025 are filled with a dielectricfiller material having a high breakdown voltage, such as Rexolite®, aproduct manufactured by C-Lec Plastics, Inc. As illustrated in FIG. 97,the wafer support pedestal 8025 consists of a grounded aluminum baseplate 9710, an aluminum electrostatic chuck plate 9720 and a cylindricalside wall 9730. Dielectric filler material 9735 fills voids between theside wall 9730 and the electrostatic chuck plate 9720. Dielectric fillermaterial 9737 fills voids between the electrostatic chuck plate 9720 andthe base plate 9710. A coaxial RF conductor 9739 carrying the RF biaspower from the RF generator 8065 (not shown in FIG. 97) is terminated ina narrow cylindrical conductive center plug 9740 that fits tightlywithin a matching conductive receptacle 9742 of the electrostatic chuckplate 9720. A wafer lift pin 9744 (one of three) extends through thepedestal 8025. The lift pin 9744 is tightly held within theelectrostatic chuck plate 9720 by a surrounding blanket 9746 of thedielectric filler material. A void 9748 that accommodates a guide 9750of the lift pin 9744 is located entirely within the base plate 9710 soas to minimize the risk of arcing within the void 9748. Referring toFIG. 98, bolt 9754 (one of several) holding the base plate 9710 and theelectrostatic chuck plate 9720 together is completely encapsulated toeliminate any voids around the bolt 9754, with dielectric layers 9756,9758 surrounding exposed portions of the bolt 9754. The foregoingfeatures have been found to enable the wafer support pedestal towithstand an RF bias voltage of over 10,000 volts without experiencingarcing.

FIG. 99 illustrates an ion implantation system including a plasmaimmersion ion implantation reactor 9910 of the type illustrated in FIGS.79, 83A, 83B, 84, 85, 88, 89A or 93. An independent source 9920 ofchamber-cleaning radicals or gases (such as fluorine-containing gases orfluorine-containing radicals like NF₃ and/or other cleaning gases suchas hydrogen-containing gases (e.g., H₂ or compounds of hydrogen) toproduce hydrogen-containing radicals or oxygen-containing gases (e.g.,O₂) is coupled to the implant reactor 9910 for use during chambercleaning operations. A post-implant anneal chamber 9930 and an ion beamimplanter 9940 are also included in the system of FIG. 99. In addition,an optical metrology chamber 9950 may also be included. Furthermore, aphotoresist pyrolization chamber 9952 may be included in the system forremoval of the photoresist mask subsequently after implant and prior toanneal. Alternatively, this may be accomplished within the plasmaimmersion implantation reactor 9910 using the RF plasma source power andoptional bias power with oxygen gas, and/or by using the independentself-cleaning source with oxygen gas.

The system of FIG. 99 may also include a wet clean chamber 9956 forcarrying out wafer cleaning. The wet clean chamber 9956 may employ suchwell known wet cleaning species as HF, for example. The wet cleanchamber 9956 may be employed for pre-implantation or post-implantationcleaning of the wafer. The pre-implantation cleaning use of the wetclean chamber 9956 may be for removing a thin native oxide that canaccumulate on the wafer between processing operations. Thepost-implantation cleaning use of the wet clean chamber 9956 may be forremoving photoresist from the wafer in lieu of the photoresist stripchamber 9952. The system of FIG. 99 may further include a second,(third, fourth or more) plasma immersion ion implantation reactor 9958of the type illustrated in FIGS. 79, 83A, 83B, 84, 85, 88, 89A or 93. Inone example, the first PIII reactor 9910 may be configured to ionimplant a first species while the second PIII reactor 9958 may beconfigured to implant a second species, so that a single PIII reactorneed not be re-configured to implant the two species in each wafer.Furthermore, the first and second species may be dopant impurities foropposite semiconductor conductivity types (e.g., boron and phosphorus),in which case the second PIII reactor 9958 may be employed in lieu ofthe beam implantation tool 9940. Or, two N-type dopants (phosphorous andarsenic) may be implanted in addition to a P-type dopant (boron), inwhich case boron implantation is carried out by the first PIII reactor9910, arsenic implantation is carried out in the ion beam tool 9940 andphosphorus implantation is carried out in the second PIII reactor 9958,for example. In another example, the 2 (or more) PIII reactors may beconfigured to implant the same species so as to increase the throughputof the system.

A wafer transfer robotic handler 9945 transfers wafers between theplasma ion implant reactor 9910, the anneal chamber 9930, the ion beamimplanter 9940, the photoresist pyrolization chamber 9952, the opticalmetrology chamber 9950, the wet clean chamber 9956 and the second PIIIreactor 9958. If the entire system of FIG. 99 is provided on a singletool or frame, the handler 9945 is a part of that tool and is supportedon the same frame. However, if some of the components of the system ofFIG. 99 are on separate tools located in separate places in a factory,then the handler 9945 is comprised of individual handlers within eachtool or frame and a factory interface that transports wafers betweentools within the factory, in the well-known manner. Thus, some or all ofthe components of the system of FIG. 99 may be provided on a single toolwith its own wafer handler 9945. Alternatively, some or all of thecomponents of the system of FIG. 99 may be provided on respective tools,in which case the wafer handler 9945 includes the factory interface.

The process controller 8075 can receive measurements of a previouslyimplanted wafer from the optical metrology chamber 9950, and adjust theimplant process in the plasma implant reactor 9910 for later wafers. Theprocess controller 8075 can use established data mining techniques forprocess correction and control. The inclusion of the ion beam implanter9940 permits the system to perform all of the ion implantation stepsrequired in semiconductor fabrication, including implantation of lightelements (such as boron or phosphorous) by the plasma ion implantreactor 9910 and implantation of heavier elements (such as arsenic) bythe ion beam implanter 9940. The system of FIG. 99 may be simplified.For example, a first version consists of only the chamber cleaningradical source 9920, the PIII reactor 9910 and the process controller8075. A second version includes the foregoing elements of the firstversion and, in addition, the optical metrology tool 9950. A thirdversion includes the foregoing elements of the second version and, inaddition, the ion beam implanter 9940 and/or the second PIII reactor9958. A fourth version includes the foregoing elements of the thirdversion and, in addition, the anneal chamber 9930.

Ion Implantation Performance of the Torroidal Source:

The plasma immersion ion implantation (PIII) reactor of FIG. 85 realizesmany advantages not found heretofore in a single reactor. Specifically,the PIII reactor of FIG. 85 has low minimum ion implant energy (becauseit has a low plasma potential), low contamination (because therecirculating plasma generally does not need to interact with chambersurfaces to provide a ground return), very good control over unwantedetching (because it exhibits low fluorine dissociation), and excellentcontrol over ion implant flux (because it exhibits a nearly linearresponse of plasma electron density to source power).

The advantage of excellent control over ion implant flux is illustratedin the graph of FIG. 100, in which electron density is plotted as afunction of source power level for the torroidal source PIII reactor ofFIG. 85 and for an inductively coupled PIII reactor of the typeillustrated in FIG. 79. Electron density is an indicator of plasma iondensity and therefore of the ion implant flux or implant dose to thewafer. The inductively coupled source of the PIII reactor of FIG. 79tends to have a highly non-linear response of electron density toapplied source power, exhibiting a sudden increase in electron densityat a threshold power level, PICP, below which the slope (response) isnegligible and above which the slope (response) is so steep thatelectron density (and therefore ion implant flux or dose) is nearlyimpossible to control to any fine degree. In contrast the torroidalsource P111 reactor of FIG. 85 has a generally linear and gradualresponse of electron density to source power level above a thresholdpower level PTH, so that ion implant flux (dose) is readily controlledto within a very fine accuracy even at very high source power level. Itshould be noted here that the plasma source power level of the torroidalsource PIII reactor of FIG. 85 is a function of the two different sourcepower generators 8055, 8056 coupled to the respective reentrant conduits8150, 8151. The source power frequency may be about 13.56 MHz, althoughthe frequency of each of the two source power generators 8055, 8056 areoffset from this frequency (13.56 MHz) by +100 kHz and −100 kHz,respectively, so that the two torroidal plasma current paths establishedby the sources 8110 and 8111 are decoupled from one another by beingde-tuned from one another by about 200 kHz. However, their power levelsmay be generally about the same. Operating frequencies are not limitedto the regime described here, and another RF frequency and frequencyoffset may be selected for the pair of RF source power generators 8055,8056.

The advantage of low fluorine dissociation of the PIII reactor of FIG.85 is important in preventing unwanted etching that can occur when afluorine-containing dopant gas, such as BF3, is employed. The problem isthat if the BF3 plasma by-products are dissociated into the simplerfluorine compounds, including free fluorine, the etch rate increasesuncontrollably. This problem is solved in the PIII reactor of FIG. 85 bylimiting the fluorine dissociation even at high power levels and highplasma density. This advantage is illustrated in the graph of FIG. 101,in which free fluorine density (an indicator of fluorine dissociation)is plotted as a function of source power for the PIII reactor of FIG. 85and for the inductively coupled reactor of FIG. 79 for the sake ofcomparison. The inductively coupled reactor of FIG. 79 exhibits anextremely sudden increase in free fluorine density above a particularsource power level, PDIS, above which the dissociation increases at avery high rate of change, and is therefore difficult to control. Incontrast, the PIII reactor of FIG. 85 exhibits generally linear andnearly negligible (very gradual) increase in free fluorine density abovea threshold source power PTH. As a result, there is very little unwantedetching during ion implantation with fluorine-containing dopant gases inthe torroidal source PIII reactor of FIG. 85. The etching is furtherminimized if the temperature of the wafer is held to a low temperature,such as below 100 degrees C., or more preferably below 60 degrees C., ormost preferably below 20 degrees C. For this purpose, the wafer pedestal8025 may be an electrostatic chuck that holds and releases the waferelectrostatically with thermal control cooling apparatus 8025 a and/orheating apparatus 8025 b that control the temperature of a semiconductorwafer or workpiece held on the top surface of the wafer support pedestal8025. Some small residual etching (such as may be realized with thetorroidal source PIII reactor of FIG. 85) is acceptable and may actuallyprevent the deposition of unwanted films on the wafer during ionimplantation. During ion implantation, some plasma by-products maydeposit as films on the wafer surface during ion implantation. This isparticularly true in cases where the implantation process is carried outat a very low ion energy (low bias voltage) and particularly with adopant gas consisting of a hydride of the dopant species (e.g., ahydride of boron or a hydride of phosphorous). In order to furtherreduce unwanted depositions that normally occur with hydride dopants(e.g., B₂H₆, PH₃), one aspect of the process is to add hydrogen and/orhelium to the dopant gas to eliminate the deposition on the surface ofthe wafer. However, the requisite etch rate to compete with such anunwanted deposition is very low, such as that exhibited by the torroidalsource PIII reactor of FIG. 85.

The advantage of a low minimum ion implant energy increases the range ofjunction depths of which the PIII reactor of FIG. 85 is capable (byreducing the lower limit of that range). This advantage is illustratedin the graph of FIG. 102, in which plasma potential is plotted as afunction of plasma source power for the torroidal source PIII reactor ofFIG. 85 and for the capacitively coupled PIII reactor of FIG. 83A, forthe sake of comparison. The plasma potential is the potential on ions atthe wafer surface due to the plasma electric field in the absence of anybias voltage on the wafer, and therefore is an indicator of the minimumenergy at which ions can be implanted. FIG. 102 shows that the plasmapotential increases indefinitely as the source power is increased in thecapacitively coupled PIII reactor of FIG. 83A, so that in this reactorthe minimum implant energy is greatly increased (the implantenergy/depth range is reduced) at high plasma density or ion implantflux levels. In contrast, above a threshold power PTH, the torroidalsource PIII reactor of FIG. 85 exhibits a very gradual (nearlyimperceptible) increase in plasma potential as source power isincreased, so that the plasma potential is very low even at high plasmasource power or ion density (high ion implant flux). Therefore, therange of plasma ion energy (ion implant depth) is much larger in thePIII reactor of FIG. 85 because the minimum energy remains very low evenat high ion flux levels.

The plasma potential in the capacitively coupled PIII reactor of FIG.83A can be reduced by increasing the source power frequency. However,this becomes more difficult as the junction depth and corresponding ionenergy is reduced. For example, to reach a plasma potential that is lessthan 500 eV (for a 0.5 kV Boron implant energy), the source powerfrequency would need to be increased well into the VHF range andpossibly above the VHF range. In contrast, the source power frequency ofthe torroidal source PIII reactor of FIG. 85 can be in the HF range(e.g., 13 MHz) while providing a low plasma potential.

A further advantage of the torroidal source PIII reactor of FIG. 85 overthe capacitively coupled source PIII reactor of FIG. 83A is that thetorroidal source PIII reactor has a thinner plasma sheath in whichproportionately fewer inelastic collisions of ions occur that tend toskew the ion implant energy distribution. This thinner sheath may benearly collisionless. In contrast, the capacitively coupled source PIIIreactor of FIG. 83A generates plasma ions in the sheath by an HF or VHFRF source that tends to produce a much thicker sheath. The thickersheath produces far more collisions that significantly skew ion energydistribution. The result is that the ion implanted junction profile isfar less abrupt. This problem is more acute at lower ion energies(shallower implanted junctions) where the skew in energy produced by thecollisions in the thicker sheath represent a far greater fraction of thetotal ion energy. The torroidal source PIII reactor of FIG. 85 thereforehas more precise control over ion implant energy and is capable ofproducing implanted junctions with greater abruptness, particularly forthe more shallow junctions that are needed for the more advanced(smaller feature size) technologies.

A related advantage of the torroidal source PIII reactor of FIG. 85 isthat it can be operated at much lower chamber pressures than thecapacitively coupled PIII reactor of FIG. 83A. The capacitively coupledPIII reactor of FIG. 83A requires a thicker sheath to generate plasmaions in the sheath, which in turn requires higher chamber pressures(e.g., 10-100 mT). The torroidal source PIII reactor of FIG. 85 does notneed to generate plasma near the sheath with bias power and for manyapplications therefore is best operated with a thinner (nearlycollisionless) sheath, so that chamber pressures can be very low (e.g.,1-3 mT). This has the advantage of a wider ion implantation processwindow in the torroidal source PIII reactor. However, as will bediscussed with reference to doping of a three dimensional structure suchas a polysilicon gate having both a top surface and vertical side walls,velocity scattering of dopant ions in the sheath enables ions to implantnot only the top surface of the polysilicon gate but also implant itsside walls. Such a process may be referred to as conformal ionimplanting. Conformal ion implanting has the advantage of doping thegate more isotropically and reducing carrier depletion at thegate-to-thin oxide interface, as will be discussed below. Therefore,some sheath thickness is desirable in order to scatter a fraction of thedopant ions away from a purely vertical trajectory so that the scatteredfraction implants into the side walls of the polysilicon gate. (Incontrast, in an ion beam implanter, such scattering is not a feature, sothat only the gate top surface is implanted.) Another advantage of aplasma sheath of finite thickness (and therefore finite collisionalcross-section) is that some very slight scattering of all the ions froma purely vertical trajectory (i.e., a deflection of only a few degrees)may be desirable in some cases to avoid implanting along an axis of thewafer crystal, which could lead to channeling or an implant that is toodeep or a less abrupt junction profile. Also, scattering of the ionsleads to placement of dopants under the polysilicon gate. This can bevery useful in optimizing CMOS device performance by controlling thedopant overlap under the poly Si gate and Source drain extension areas,as will be discussed later in this specification in more detail.

The low contamination exhibited by the torroidal source PIII reactor ofFIG. 85 is due primarily to the tendency of the plasma to not interactwith chamber surfaces and instead oscillate or circulate in thetorroidal paths that are generally parallel to the chamber surfacesrather than being towards those surfaces. Specifically, the pairtorroidal paths followed by the plasma current are parallel to thesurfaces of the respect reentrant conduits 8150, 8151 of FIG. 85 andparallel to the interior surface of the ceiling 8015 and of the wafersupport pedestal 8025. In contrast, the plasma source power generateselectric fields within the capacitively coupled PIII reactor of FIG. 83Athat are oriented directly toward the ceiling and toward the chamberwalls.

In the torroidal source PIII reactor of FIG. 85, the only significantelectric field oriented directly toward a chamber surface is produced bythe bias voltage applied to the wafer support pedestal 8025, but thiselectric field does not significantly generate plasma in the embodimentof FIG. 85. While the bias voltage can be a D.C. (or pulsed D.C.) biasvoltage, in the embodiment of FIG. 85 the bias voltage is an RF voltage.The frequency of the RF bias voltage can be sufficiently low so that theplasma sheath at the wafer surface does not participate significantly inplasma generation. Thus, plasma generation in the torroidal source PIIIreactor of FIG. 85 produces only plasma currents that are generallyparallel to the interior chamber surfaces, and thus less likely tointeract with chamber surfaces and produce contamination.

Further reduction of metal contamination of ion implantation processesis achieved by first depositing a passivation layer on all chambersurfaces prior to performing the ion implantation process. Thepassivation layer may be a silicon-containing layer such as silicondioxide, silicon nitride, silicon, silicon carbide, silicon hydride,silicon fluoride, boron or phosphorous or arsenic doped silicon, boronor phosphorous or arsenic doped silicon carbide, boron or phosphorous orarsenic doped silicon oxide. Alternatively, the passivation may be afluorocarbon or hydrocarbon or hydrofluorocarbon film. Compounds ofgermanium may also be used for passivation. Alternatively, thepassivation layer may be a dopant-containing layer such as boron,phosphorous, arsenic or antimony formed by decomposition of a compoundof the dopant precursor gas, such as BF₃, B₂H₆, PF₃, PF₅, PH₃, AsF₃, ofAsH₃. It may be advantageous to form a passivation layer with a sourcegas or source gas mixture using gas(es) similar to that or those thatare to be used in the subsequent plasma immersion implantation processstep. (This may reduce unwanted etching of the passivation layer by thesubsequent implant process step.) Alternatively, it may be advantageousto combine the fluoride and the hydride of a particular gas to minimizethe fluorine and/or hydrogen incorporated in the passivation layer, forexample, BF₃+B₂H₆, PH₃+PF₃, AsF₃+AsH₃, SiF₄+SiH₄, or GeF₄+GeH₄.

While the RF bias frequency of the torroidal source PIII reactor of FIG.85 is sufficiently low to not affect plasma generation by the plasmasource power applicators 8110, 8111, it is also sufficiently low topermit the ions in the plasma sheath to follow the sheath oscillationsand thereby acquire a kinetic energy of up to the equivalent to the fullpeak-to-peak voltage of the RF bias power applied to the sheath,depending upon pressure and sheath thickness. This reduces the amount ofRF bias power required to produce a particular ion energy or implantdepth. On the other hand, the RF bias frequency is sufficiently high toavoid significant voltage drops across dielectric layers on the wafersupport pedestal 8025, on chamber interior walls and on the waferitself. This is particularly important in ion implantation of veryshallow junctions, in which the RF bias voltage is correspondinglysmall, such as about 150 volts for a 100 Angstrom junction depth (forexample). An RF voltage drop of 50 volts out of a total of 150 voltsacross the sheath (for example) would be unacceptable, as this would bea third of the total sheath voltage. The RF bias frequency is thereforesufficiently high to reduce the capacitive reactance across dielectriclayers so as to limit the voltage drop across such a layer to less thanon the order of 10% of the total RF bias voltage. A frequencysufficiently high meet this latter requirement while being sufficientlylow for the ions to follow the sheath oscillations is in the range of100 kHz to 10 MHz, and more optimally in the range of 500 kHz to 5 MHz,and most optimally about 2 MHz. One advantage of reducing capacitivevoltage drops across the wafer pedestal is that the sheath voltage canbe more accurately estimated from the voltage applied to the pedestal.Such capacitive voltage drops can be across dielectric layers on thefront or back of the wafer, on the top of the wafer pedestal and (in thecase of an electrostatic chuck) the dielectric layer at the top of thechuck.

Ion implantation results produced by the torroidal source PIII reactorof FIG. 85 compare favorably with those obtained with a conventionalbeam implanter operated in drift mode, which is much slower than thePIII reactor. Referring to FIG. 103, the curves “A” and “a” are plots ofdopant (boron) volume concentration in the wafer crystal as a functionof depth for boron equivalent energies of 0.5 keV. (As will be discussedbelow, to achieve the same ion energy as the beam implanter, the biasvoltage in the PIII reactor must be twice the acceleration voltage ofthe beam implanter.) Even though the PIII reactor (curve “A”) is fourtimes faster than the beam implanter (curve “B”), the implant profile isnearly the same, with the same junction abruptness of about 3 nanometers(change in junction depth) per decade (of dopant volume concentration)and junction depth (about 100 Angstroms). Curves “B” and “b” compare thePIII reactor results (“B”) with those of a conventional beam implanter(“b”) at boron equivalent energies of 2 keV, showing that the junctionabruptness and the junction depth (about 300 Angstroms) is the same inboth cases. Curves “C” and “c” compare the PIII reactor results (“C”)with those of a conventional beam implanter (“c”) at boron equivalentenergies of 3.5 keV, showing that the junction depth (about 500Angstroms) is the same in both cases.

FIG. 103 compares the PIII reactor performance with the conventionalbeam implanter operated in drift mode (in which the beam voltagecorresponds to the desired junction depth). Drift mode is very slowbecause the beam flux is low at such low beam energies. This can beaddressed by using a much higher beam voltage and then decelerating thebeam down to the correct energy before it impacts the wafer. Thedeceleration process is not complete, and therefore leaves an energy“contamination” tail (curve “A” of FIG., 104) which can be reduced byrapid thermal annealing to a better implant profile with greaterabruptness (curve “B” of FIG. 104). Greater activated implanted dopantconcentration, however, can be achieved using a dynamic surfaceannealing process employing localized melting or nearly meltingtemperatures for very short durations. The dynamic surface annealingprocess does not reduce energy contamination tails, such as the energycontamination tail of curve “C” of FIG. 105. In comparison, thetorroidal source PIII reactor of FIG. 85 needs no deceleration processsince the bias voltage corresponds to the desired implant depth, andtherefore has no energy contamination tail (curve “D” of FIG. 105).Therefore, the PIII reactor can be used with the dynamic surface annealprocess to form very abrupt ultra shallow junction profile, while theconventional beam implanter operating in deceleration mode cannot. Thedynamic surface annealing process consists of locally heating regions ofthe wafer surface to nearly (e.g., within 100 to 50 degrees of) itsmelting temperature for very short durations (e.g., nano-seconds to tensof milliseconds) by scanning a laser beam or a group of laser beamsacross the wafer surface.

FIG. 106 illustrates how much greater a dopant concentration can beattained with the dynamic surface annealing process. Curve “A” of FIG.106 illustrates the wafer resistivity in Ohms per square as a functionof junction depth using a beam implanter and a rapid thermal anneal ofthe wafer at 1050 degrees C. The concentration of dopant reached 10²⁰per cubic centimeter. Curve “B” of FIG. 106 illustrates the waferresistivity in Ohms per square as a function of junction depth using thetorroidal source PIII reactor of FIG. 85 and a dynamic surface annealprocess after implanting at a temperature of 1300 degrees C. Theconcentration of the dopant reached 5×10²⁰ following the dynamic surfaceannealing, or about five times that achieved with rapid thermalannealing. FIG. 107 illustrates how little the implanted dopant profilechanges during dynamic surface annealing. Curve “A” of FIG. 107 is thedopant distribution prior to annealing while curve “B” of FIG. 107 isthe dopant distribution after annealing. The dynamic surface annealingprocess causes the dopant to diffuse less than 10 Å, while it does notadversely affect the junction abruptness, which is less than 3.5nm/decade. This tendency of the dynamic surface annealing process tominimize dopant diffusion facilitates the formation of extremely shallowjunctions. More shallow junctions are required (as source-to-drainchannel lengths are decreased in higher speed devices) in order to avoidsource-to-drain leakage currents. On the other hand, the shallowerjunction require much higher active dopant concentrations (to avoidincreased resistance) that can best be realized with dynamic surfaceannealing. As discussed elsewhere in this specification, junction depthcan be reduced by carrying out a wafer amorphization step in which thewafer is bombarded with ions (such as silicon or germanium ions) tocreate lattice defects in the semiconductor crystal of the wafer. Wehave implanted and annealed junctions having a high dopant concentrationcorresponding to a low resistivity (500 Ohms per square), an extremelyshallow junction depth (185 Å) and a very steep abruptness (less than 4nm/decade). In some cases, the depth of the amorphizing or ionbombardment process may extend below the dopant implant junction depth.For example, amorphization using SiF4 gas and a 10 kV peak-to-peak biasvoltage in the PIII reactor of FIG. 85 forms an amorphized layer to adepth of about 150 Angstroms, while dopant (boron) ions acceleratedacross a 1000 peak-to-peak volt sheath (bias) voltage implant to a depthof only about 100 Angstroms.

FIG. 108 illustrates the bias voltage for the torroidal source PIIIreactor (left hand ordinate) and the beam voltage for the ion beamimplanter (right hand ordinate) as a function of junction depth. ThePIII reactor and the beam implanter produce virtually identical resultsprovided the PIII reactor bias voltage is twice the beam voltage.

WORKING EXAMPLES

A principal application of a PIII reactor is the formation of PNjunctions in semiconductor crystals. FIGS. 109 and 110 illustratedifferent stages in the deposition of dopant impurities in thefabrication of a P-channel metal oxide semiconductor field effecttransistor (MOSFET). Referring first to FIG. 109, a region 9960 of asemiconductor (e.g., silicon) wafer may be doped with an N-typeconductivity impurity, such as arsenic or phosphorus, the region 9960being labeled “n” in the drawing of FIG. 109 to denote its conductivitytype. A very thin silicon dioxide layer 9962 is deposited on the surfaceof the wafer including over n-type region 9960. A polycrystallinesilicon gate 9964 is formed over the thin oxide layer 9962 from ablanket polysilicon layer that has been doped with boron in the PIIIreactor. After formation of the gate 9964, p-type dopant is implanted inthe PIII reactor to form source and drain extensions 9972 and 9973.Spacer layers 9966 of a dielectric material such as silicon dioxideand/or silicon nitride (for example) are formed along two oppositevertical sides 9964 a, 9964 b of the gate 9964. Using the PIII reactorof FIG. 85 with a process gas consisting of BF3 or B2H6 (for example),boron is implanted over the entire N-type region 9960. The spacer layersmask their underlying regions from the boron, so that P-typeconductivity source and drain contact regions 9968, 9969 are formed oneither side of the gate 9964, as shown in FIG. 110. This step is carriedout with a boron-containing species energy in the range of 2 to 10 kVppon the RF bias voltage (controlled by the RF bias power generator 8065of FIG. 85). In accordance with the example of FIG. 108, the RF biasvoltage on the wafer pedestal 8025 in the PIII reactor of FIG. 85 istwice the desired boron energy. The implantation is carried out for asufficient time and at a sufficient ion flux or ion density (controlledby the RF source power generators 8055, 8056 of FIG. 85) to achieve asurface concentration of boron exceeding 5×10¹⁵ atoms per squarecentimeter. The concentration of boron in the gate 9964 is thenincreased to 1×10¹⁶ atoms per square centimeter by masking the sourceand drain contacts 9968, 9969 (by depositing a layer of photoresistthereover, for example) and carrying out a further (supplementary)implantation step of boron until the concentration of boron in the gate9964 reaches the desired level (1×10¹⁶ atoms/cubic centimeter). Thesource and drain contacts 9968, 9969 are not raised to the higher dopantconcentration (as is the gate 9964) because the higher dopantconcentration may be incompatible with formation of a metal silicidelayer (during a later step) over each contact 9968, 9969. However, thegate 9964 must be raised to this higher dopant concentration level inorder to reduce carrier depletion in the gate 9964 near the interfacebetween the gate 9964 and the thin silicon dioxide layer 9962. Suchcarrier depletion in the gate would impede the switching speed of thetransistor. The dopant profile in the gate must be highly abrupt inorder attain a high dopant concentration in the gate 9964 near the thinoxide layer 9962 without implanting dopant into the underlying thinoxide layer 9962 or into the source-to-drain channel underlying the thinoxide layer 9962. Another measure that can be taken to further enhancegate performance and device speed is to raise the dielectric constant ofthe thin silicon dioxide layer 9962 by implanting nitrogen in the thinsilicon dioxide layer 9962 so that (upon annealing) nitrogen atomsreplace oxygen atoms in the layer 9962, as will be described later inthis specification. A further measure for enhancing gate performance isconformal implanting in which dopant ions that have been deflected fromtheir vertical trajectory by collisions in the plasma sheath over thewafer surface are able to implant into the vertical side walls of thegate 9964. This further increases the dopant concentration in the gate9964 near the interface with the thin oxide layer 9962, and provide amore uniform and isotropic dopant distribution within the gate. A yetfurther measure for enhancing gate performance for gates of N-channeldevices implanted with arsenic is to implant phosphorus during thesupplementary implant step using the PIII reactor. The phosphorus islighter than arsenic and so diffuses more readily throughout thesemiconductor crystal, to provide less abrupt junction profile in thesource drain contact areas.

The depth of the ion implantation of the source and drain contacts 9968,9969 may be in the range of 400 to 800 Å. If the gate 9964 is thinnerthan that, then the gate 9964 must be implanted in a separateimplantation step to a lesser depth to avoid implanting any dopant inthe thin oxide layer 9962 below the gate 9964. In order to avoiddepletion in the region of the gate 9964 adjacent the thin oxide layer9962, the implantation of the gate must extend as close to thegate/oxide interface as possible without entering the thin oxide layer9962. Therefore, the implant profile of the gate must have the highestpossible abruptness (e.g., 3 nm/decade or less) and a higher dopant dose(i.e., 1×10¹⁶ atoms/cm²).

Referring now to FIG. 110, source and drain extensions 9972, 9973 aretypically formed before depositing and forming the spacer layers 9966 ofFIG. 109. The extensions layers are formed by carrying out a moreshallow and light implant of boron over the entire region 9960.Typically, the junction depth of the source and drain extensions is onlyabout 100 to 300 Angstroms and the implant dose is less than 5×10¹⁵atoms/square centimeter. This implant step, therefore, has little effecton the dopant profiles in the gate 9964 or in the source and draincontacts 9968, 9969, so that these areas need not be masked during theimplantation of the source and drain extensions 9972, 9973. However, ifmasking is desired, then it may be carried out with photoresist. Thesource and drain extensions are implanted at an equivalent boron energyof 0.5 kV, requiring a 1.0 kvpp RF bias voltage on the wafer pedestal8025 of FIG. 85.

The same structures illustrated in FIGS. 109 and 110 are formed in thefabrication of an N-channel MOSFET. However, the region 9960 isinitially doped with a P-type conductivity such as boron and istherefore a P-type conductivity region. And, the implantation of thegate 9964 and of the source and drain contacts 9968, 9969 (illustratedin FIG. 109) is carried out in a beam implanter (rather than in a PIIIreactor) with an N-type conductivity impurity dopant such as arsenic.Furthermore, the supplementary implantation of the gate 9964 that raisesits dopant dose concentration to 1×10¹⁶ atoms/cm² is carried out in thePIII reactor with phosphorus (rather than arsenic) using aphosphorus-containing process gas. Phosphorus is preferred for thislatter implantation step because it diffuses more uniformly thanarsenic, and therefore enhances the quality of the N-type dopant profilein the gates 9964 of the N-channel devices. The ion beam voltage is inthe range of 15-30 kV for the arsenic implant step (simultaneousimplanting of the N-channel source and drain contacts 9968, 9969 and ofthe N-channel gates 9964), and is applied for a sufficient time to reacha dopant surface concentration exceeding 5×10¹⁵ atoms per cubiccentimeter. The supplementary gate implant of phosphorus is carried outat an ion beam voltage in the range of only 2-5 kV for a sufficient timeto raise the dopant surface concentration in the N-channel gates to1×10¹⁶ atoms/cubic cm.

The implantation steps involving phosphorus and boron are advantageouslycarried out in the PIII reactor rather than an ion beam implanterbecause the ion energies of these light elements are so low that ionflux in a beam implanter would be very low and the implant times wouldbe inordinately high (e.g., half and hour per wafer). In the PIIIreactor, the source power can be 800 Watts at 13.56 MHz (with the 200kHz offset between the two torroidal plasma currents as describedabove), the implant step being carried out for only 5 to 40 seconds perwafer.

The sequence of ion implantation steps depicted in FIGS. 109 and 110 maybe modified, in that the light shallow source and drain extensionimplant step of FIG. 110 may be carried out before or after formation ofthe spacer layer 9966 and subsequent heavy implantation of the contacts9968, 9969 and gate 9964. When extension implants are done after thespacer layer 9966 is formed, the spacer layer 9966 must be removedbefore the extension implants are performed.

One example of a process for fabricating complementary MOSFETS (CMOSFETs) is illustrated in FIG. 111. In the first step (block 9980), theP-well and N-well regions of the CMOS device are implanted in separatesteps. Then, a blanket thin gate oxide layer and an overlying blanketpolysilicon gate layer are formed over the entire wafer (block 9981 ofFIG. 111). The P-well regions are masked and the N-well regions are leftexposed (block 9982). The portions of the polysilicon gate layer lyingin the N-well regions are then implanted with boron in a PIII reactor(block 9983). The P-channel gates (9964 in FIG. 109) are thenphotolithographically defined and etched, to expose portions of thesilicon wafer (block 9984). Source and drain extensions 9972, 9973 ofFIG. 109 self-aligned with the gate 9964 are then formed by ionimplantation of boron using the PIII reactor (block 9985). A so-called“halo” implant step is then performed to implant an N-type dopant underthe edges of each P-channel gate 9964 (block 9986). This is done byimplanting arsenic using an ion beam tilted at about 30 degrees from avertical direction relative to the wafer surface and rotating the wafer.Alternatively, this step may be accomplished by implanting phosphorus inthe PIII reactor using a chamber pressure and bias voltage conducive toa large sheath thickness to promote collisions in the sheath that divertthe boron ions from a vertical trajectory. Then, the spacer layers 9986are formed over the source drain extensions 9972, 9973 (block 9987) andboron is then implanted at a higher energy to form the deep source draincontacts 9969 (block 9988), resulting in the structure of FIG. 110. Thereverse of step 9982 is then performed by masking the N-well regions(i.e., the P-channel devices) and exposing the P-well regions (block9992). Thereafter steps 9993 through 9998 are performed that correspondto steps 9983 through 9988 that have already been described, except thatthey are carried out in the P-well regions rather than in the N-wellregions, the dopant is Arsenic rather than Boron, and a beam line ionimplanter is employed rather than a PIII reactor. And, for the N-channeldevice halo implant of block 9996 (corresponding to the P-channel devicehalo implant of block 9986 described above), the dopant is a P-typedopant such as boron. In the case of the N-channel devices implanted insteps 9993 through 9998, a further implant step is performed, namely asupplemental implant step (block 9999) to increase the dose in thepolysilicon gate as discussed above in this specification. In thesupplemental implantation step of block 9999, phosphorus is the N-typedopant impurity and is implanted using a PIII reactor rather than a beamimplanter (although a beam implanter could be employed instead).

As noted above, the process may be reversed so that the gate 9964 andsource and drain contacts 9968, 9969 are implanted before the source anddrain extensions 9972, 9973.

After all ion implantations have been carried out, the wafer issubjected to an annealing process such as spike annealing using rapidthermal processing (RTP) and/or the dynamic surface annealing (DSA)process discussed earlier in this specification. Such an annealingprocess causes the implanted dopant ions, most of which came to rest ininterstitial locations in the crystal lattice, to move to atomic sites,i.e., be substituted for silicon atoms originally occupying those sites.More than one annealing step can be used to form the pmos and nmosdevices and these steps can be inserted in the process flow asappropriate from activation and diffusion point of view.

The foregoing ion implantation processes involving the lighter atomicelements (e.g., boron and phosphorus) are carried out using a PIIIreactor in the modes described previously. For example, the bias powerfrequency is selected to maximize ion energy while simultaneouslyproviding low impedance coupling across dielectric layers. How this isaccomplished is described above in this specification.

The ion implantation processes described above are enhanced by otherprocesses. Specifically, in order to prevent channeling and in order toenhance the fraction of implanted ions that become substitutional uponannealing, the semiconductor wafer crystal may be subjected to an ionbombardment process that partially amorphizes the crystal by creatingcrystal defects. The ions employed should be compatible with the wafermaterial, and may be formed in the PIII reactor in a plasma generatedfrom one or more of the following gases: silicon fluoride, siliconhydride, germanium fluoride, germanium hydride, Xenon, Argon, or carbonfluoride (i.e., tetrafluoromethane, octafluorocyclobutane, etc) orcarbon hydride (i.e., methane, acetylene, etc) or carbonhydride/fluoride (i.e., tetrafluoroethane, difluoroethylene, etc.)gases. One advantage of the PIII reactor is that its implant processesare not mass selective (unlike an ion beam implanter). Therefore, duringion implantation of a dopant impurity such boron, any other element mayalso be implanted simultaneously, regardless of ion mass in the PIIIreactor. Therefore, unlike an ion beam implanter, the PIII reactor iscapable of simultaneously implanting a dopant impurity while carryingout an amorphizing process. This may be accomplished using a BF3 gas (toprovide the dopant ions) mixed with an SiF4 gas (to provided theamorphizing bombardment ion species). Such a simultaneous ionimplantation process is referred to as a co-implant process. Theamorphization process may also be carried out sequentially with thedoping process. In addition to amorphization, simultaneous implants ofdopant and non-dopant atoms such as Fluorine, Germanium, Carbon or otherelements are done to change the chemistry of the Silicon wafer. Thischange in chemistry can help in increasing dopant activation andreducing dopant diffusion.

Another process that can be carried out in the PIII reactor is a surfaceenhancement process in which certain ions are implanted in order toreplace other elements in the crystal. One example of such a surfaceenhancement process is nitrodization. In this process, the dielectricconstant of the thin silicon dioxide layer 9962 is increased (in orderto increase device speed) by replacing a significant fraction of theoxygen atoms in the silicon dioxide film with nitrogen atoms. This isaccomplished in the PIII reactor by generating a plasma from anitrogen-containing gas, such as ammonia, and implanting the nitrogenatoms into the silicon dioxide layer 9962. This step may be performed atany time, including before, during and/or after the implantation of thedopant impurity species. If the nitrodization process is performed atleast partially simultaneously with the dopant ion implant step, thenthe nitrodization process is a co-implant process. Since the ionimplantation process of the PIII reactor is not mass selective, theco-implant process may be carried out with any suitable species withoutrequiring that it atomic weight be the same as or related to the atomicweight of the dopant implant species. Thus, for example, the dopantspecies, boron, and the surface enhancement species, nitrogen, havequite different atomic weights, and yet they are implantedsimultaneously in the PIII reactor. Typically nitrodization is performedwithout implanting dopant atoms.

A further process related to ion implantation is surface passivation. Inthis process, the interior surfaces of the reactor chamber, includingthe walls and ceiling, are coated with a silicon-containing passivationmaterial (such as silicon dioxide or silicon nitride or silicon hydride)prior to the introduction of a production wafer. The passivation layerprevents the plasma from interacting with or sputtering any metalsurfaces within the plasma reactor. The deposition the passivation layeris carried out by igniting a plasma within the reactor from a siliconcontaining gas such as silane mixed with oxygen, for example. Thispassivation step, combined with the low-contamination torroidal sourcePIII reactor of FIG. 85, has yielded extremely low metal contaminationof a silicon wafer during ion implantation, about 100 times lower thanthat typically obtained in a conventional beam implanter.

Upon completion of the ion implantation process, the passivation layeris removed, using a suitable etchant gas such as NF₃ which may becombined with a suitable ion bombardment gas source such as argonoxygen, or hydrogen. During this cleaning step, the chamber surfaces maybe heated to 60 degrees C. or higher to enhance the cleaning process. Anew passivation layer is deposited before the next ion implantationstep.

Alternatively, a new passivation layer may be deposited beforeimplanting a sequence of wafers, and following the processing of thesequence, the passivation layer and other depositions may be removedusing a cleaning gas.

FIG. 112 is a flow diagram showing the different options of combiningthe foregoing ion implantation-related processes with the dopantimplantation processes of FIG. 111. A first step is cleaning the chamberto remove contamination or to remove a previously deposited passivationlayer (block 9001 of FIG. 112). Next, a passivation layer of silicondioxide, for example, is deposited over the interior surfaces of thechamber (block 9002) prior to the introduction of the wafer to beprocessed. Next, the wafer is introduced into the PIII reactor chamberand may be subjected to a cleaning or etching process to remove thinoxidation layers that may have accumulated on the exposed semiconductorsurfaces in the brief interim since the wafer was last processed (block9003). A pre-implant wafer amorphizing process may be carried out (block9004) by ion-bombarding exposed surfaces of the wafer with silicon ions,for example. A pre-implant surface enhancement process may also becarried out (block 9005) by implanting a species such as nitrogen intosilicon dioxide films. The dopant implantation process may then becarried out (block 9006). This step is an individual one of the boron orphosphorus implant steps illustrated in the general process flow diagramof FIG. 111. During the dopant implant process of block 9006, other ionsin addition to the dopant ions may be implanted simultaneously in aco-implant process (block 9007). Such a co-implant process (9007) may bean amorphizing process, a light etch process that prevents accumulationof plasma by-products on the wafer surface, enhancing dopant activationand reducing dopant diffusion, or surface enhancement process. Aftercompletion of the dopant ion implant process (9006) and any co-implantprocess (9007), various post implant processes may be carried out. Suchpost implant processes may include a surface enhancement process (block9008). Upon completion of all implant steps (including the step of block9008), an implant anneal process is carried out (block 9012) afterremoving any photo-resist mask layers on the wafer in the precedingwafer clean step of block 9009. This anneal process can be a dynamicsurface anneal in which a laser beam (or several laser beams) arescanned across the wafer surface to locally heat the surface to nearlymelting temperature (about 1300 degrees C.) or to melting temperature,each local area being heated for an extremely short period of time(e.g., on the order of nanoseconds to tens of milliseconds). Other postimplant processes carried out after the anneal step of block 9112 mayinclude a wafer cleaning process (block 9009) to remove layers of plasmaby-products deposited during the ion implantation process, deposition ofa temporary passivation coating on the wafer to stabilize the wafersurface (block 9010) and a chamber cleaning process (block 9011),carried out after removal of the wafer from the PIII reactor chamber,for removing a previously deposited passivation layer from the chamberinterior surfaces.

Fabrication of SOI Structures by Plasma Immersion Ion Implantation:

The problem of plasma contamination due to plasma interaction withinterior chamber surfaces is solved by employing a plasma immersion ionimplantation reactor having a torroidal plasma source. In this source,the plasma is generated as an RF oscillatory recirculating torroidal ioncurrent. The torroidal plasma has a very low plasma potential relativeto the interior reactor chamber surfaces, and therefore has relativelylittle interaction with those surfaces. The result is very littlecontamination in the plasma immersion ion implantation process.

The problem of attaining a high quality SOI silicon surface layerrelatively free of implanted oxygen atoms is solved by variouscombinations of several special measures carried out after (or during)implantation of the oxygen “box” beneath the silicon surface layer: (a)implanting the silicon surface layer with additional silicon atoms, (b)implanting the silicon surface layer with an oxygen getter, such ashydrogen, (c) quickly implanting a low-density defect layer below theimplanted oxygen “box” to draw the box down from the silicon surfacelayer during annealing, and (d) by one of several plasma immersion ionimplantation/deposition processes in which silicon and/or an oxygengetter such as hydrogen are implanted in the silicon surface layer whilesilicon is epitaxially deposited on the surface and/or while the surfaceis treated with hydrogen to enhance the silicon epitaxial depositionprocess.

The foregoing processes are followed by an annealing step to produce astochiometric silicon dioxide layer in the oxygen-implanted “box”beneath the silicon surface layer, while drawing the “box” down to theimplanted damage layer. This anneal step also follows the implantationof silicon and/or hydrogen in the silicon surface layer, so that theanneal step promotes substitution of the implanted silicon into thecrystal lattice and/or boiling out of oxygen atoms captured by theimplanted hydrogen. This anneal step may be carried out before theepitaxial silicon deposition is performed, or may be carried outafterwards. The anneal step may be a rapid thermal anneal (using radiantheat) or a dynamic surface anneal (using a laser to raise the surfacetemperature to near boiling) or may be carried out conventionally in afurnace.

Referring to the SOI fabrication process of FIG. 113, a semiconductorwafer is placed in the plasma immersion ion implantation reactor of FIG.85 and is heated to about 600 degrees C. (block 5010). The wafer may beheated either by radiant heating (e.g., a halogen light source) whileinsulating the wafer from the wafer support pedestal. Alternatively, thewafer may be heated by maintaining thermal conductance between the waferand the wafer support pedestal and heating the wafer support pedestal.An oxygen plasma precursor gas, such as water vapor, is introduced intothe reactor of FIG. 85 to form a plasma of oxygen atoms (and OHmolecules) so that oxygen is implanted into the semiconductor wafer(block 5012 of FIG. 113). The bias voltage on the wafer support pedestalcan be set to a maximum value to make the oxygen implantation depth asgreat as possible. As explained above, it is desirable for the oxygen tobe implanted in a “box” at a sufficient implant depth below thesubstrate surface to leave a nearly pure (unimplanted) thin siliconsurface layer as the silicon-on-insulator layer (of about 3 nm inthickness or larger, for example). This thickness can be later augmentedby epitaxial silicon deposition. However, in a plasma immersion ionimplantation, the implant depth may be insufficient to leave an“unimplanted” silicon layer at the substrate surface. This is becausethe maximum bias voltage that can be applied to the wafer supportpedestal is limited by a number of factors, and in particular by theneed to avoid arcing in the plasma. As a result, a significant amount ofoxygen atoms are implanted in the silicon surface layer. The presence ofsuch impurities can render the silicon surface unsuitable for epitaxialdeposition of silicon. Therefore, prior to the epitaxial deposition ofsilicon on the substrate surface, special steps (block 5014 of FIG. 113)are taken to reduce the concentration of oxygen in the silicon surfacelayer below a certain threshold under which high quality epitaxialsilicon deposition can be achieved. Then, the silicon epitaxialdeposition is performed (block 5016 of FIG. 113) after the siliconsurface layer has been improved in the preceding step of block 5014.

A sub-process for carrying out the oxygen removal step of block 5014 isillustrated in FIG. 114. Silicon is implanted within the thin siliconsurface layer to enrich the silicon content of this layer (block 5022).The surface of the silicon substrate may be treated with hydrogen toremove any native oxide (block 5023). This step can improve thecrystalline quality of the substrate surface by removing any nativeoxide and filling some dangling silicon bonds at the surface withhydrogen. Hydrogen may also be implanted within the thin silicon surfacelayer as an oxygen-getter (block 5024). The implantation of silicon andhydrogen and the surface treatment with hydrogen may be accomplishedtogether by forming a plasma of silane, so that a mixture of hydrogenand silicon ions are implanted together in the substrate. The foregoingsteps (blocks 5022, 5023, 5024) can be effectively carried out when thewafer temperature is held at about 600 degrees C. These steps have anumber of salutary effects. The implanted hydrogen atoms can combinewith oxygen atoms in the silicon surface layer and then be evaporatedout of the crystal lattice during a later high temperature annealingstep, thereby reducing the oxygen concentration in the silicon surfacelayer. Also, the implanted silicon atoms in the silicon surface layerincrease the concentration of silicon relative to oxygen in the siliconsurface layer.

A damage layer is formed (block 5020 of FIG. 114) generally coincidingwith the ion implantation profile of the oxygen atoms implanted in theplasma immersion ion implantation step (i.e., below the thin siliconsurface layer). This latter step may be carried out at room temperature.The silicon wafer is then annealed at a very high temperature such as1300 degrees C. for a long time such as 6 hours (block 5026). Thisannealing step produces a number of effects: the profile of theimplanted oxygen layer is narrowed. This is because the implanted oxygenatoms nearer the top and bottom edges of the implanted oxygen layer(regions of lower oxygen concentration) migrate toward the center of theimplanted oxygen layer having the highest oxygen concentration by aprocess known as Oswald ripening. This narrowing of the implanted oxygenprofile draws oxygen atoms away from the silicon surface layer; and, theimplanted oxygen atoms, having migrated to the higher oxygenconcentration region at the center of the implanted oxygen profile,become substitutional in the crystal lattice to form a silicon dioxidelayer beneath the silicon surface layer.

In a variation of the foregoing process, the reduction in oxygenconcentration in the thin silicon surface layer is performed at leastpartially contemporaneously with the silicon epitaxial depositionprocess. This variation is illustrated in FIG. 115. In FIG. 115, thefirst step is to heat the wafer as before (block 5030 of FIG. 115). Thenext step is the oxygen ion implantation step (block 5032 of FIG. 115)which can be carried out in the plasma immersion ion implantation(“P3i”) reactor of FIG. 85. Then, in block 5034, steps are taken toreduce the concentration of oxygen in the silicon surface layer, atleast some of these steps being performed simultaneously with siliconepitaxial deposition. Block 5034 of FIG. 115 is amplified in FIG. 116.FIG. 116 includes key steps from the process of FIG. 114 (i.e., blocks5020, 5023, 5024 and 5026), except that during the silicon implantationstep (block 5022), the wafer bias is adjusted so that only a portion ofthe silicon atoms are implanted, the remainder reaching the substratesurface with a kinetic energy suitable for epitaxial deposition. As aresult, silicon ion implantation occurs simultaneously with epitaxialsilicon deposition in the modified step of block 5022. These steps arefollowed by the formation of the damage layer (block 5020) with animplantation profile or depth distribution corresponding to that of theimplanted oxygen. Then, the wafer is heated in an anneal process (block5026) which may be carried out at a very high temperature (e.g., 1300degrees C.) for a relatively long time (e.g., 4-6 hours). The annealstep has the effect of effecting the narrowing of the implanted oxygenprofile, and of substituting implanted silicon atoms in the crystallattice of the silicon surface layer.

One aspect of the foregoing process of FIG. 116 is illustrated in FIGS.117A-117D. Following plasma immersion ion implantation of oxygen, thesilicon substrate of FIG. 117A has a Gaussian-shaped oxygen depthdistribution illustrated in FIG. 117B. The Gaussian oxygen profile ofFIG. 117B tails off near the surface layer 5140, giving rise to asignificant concentration of oxygen (greater than 10¹⁸ cm−3) in the 3nm-thick silicon surface layer 5140 of FIG. 117A. A damage layer 5142(denoted by shading in FIG. 117C) is formed by ion implantation of asuitable species such as silicon or oxygen, with an implantation profilegenerally conforming with the oxygen implantation profile. An ion beamtool can be employed to form the deep damage layer 5142. However, therequired concentration of implanted atoms in the deep damage layer 5142is relatively small, so that the beam implantation step is relativelyquick. Annealing causes the implanted oxygen atoms to migrate toward thecenter or peak of the oxygen implantation profile due to the Oswaldripening effect, which necessarily draws oxygen atoms away from thesurface layer 5140 (thereby improving the quality of the surface layer5140). This produces the narrowed oxygen distribution of FIG. 117D. Thisnarrowing of the oxygen profile removes much of the oxygen from thesilicon surface layer 5140. The Oswald ripening effect arises from themigration of oxygen atoms from the low oxygen concentration layers andsmall SiO2 precipitates to the high oxygen concentration layers andlarge SiO2 precipitates.

Another aspect of the process of FIG. 115 is illustrated in FIGS. 118Aand 118B. FIG. 118A illustrates how silane gas in the plasma immersionion implantation reactor of FIG. 85 dissociates into hydrogen andsilicon ions, which impinge on the substrate surface. FIG. 118Aindicates that, at a suitable wafer bias voltage, some of the siliconatoms are implanted in the silicon surface layer 5140 while theremainder accumulate on the substrate surface to form an epitaxialsilicon layer 5144. The lighter hydrogen atoms are implanted below thesubstrate surface. The wafer bias voltage is selected to produce asilicon ion energy distribution that yields the silicon ion distributionillustrated in FIG. 118B, in which a fraction of the silicon ions areimplanted while the remainder accumulate on the substrate surface in anepitaxial silicon layer 5144.

FIG. 119 illustrates a modification of the process of FIG. 113 in whichthe formation of the deep damage layer and the migration of the oxygenatoms downward toward the high oxygen concentration layers at the oxygenprofile center (i.e., narrowing of the oxygen profile) is carried out tocompletion before the other steps. FIGS. 120A through 120F depict thedistribution of implanted species in the substrate at respective stagesof the process of FIG. 119. In the first step of FIG. 119, (block 5210),a semiconductor wafer is placed in the torroidal source plasma immersionion implantation reactor of FIG. 85 and heated to a suitabletemperature. This temperature is sufficient to avoid a loss of thesemiconductor crystal lattice structure that would otherwise occur dueto ion bombardment damage during ion implantation. In the case of asemiconductor silicon wafer, this temperature is about 600 degrees C.

A plasma containing oxygen ions is generated in the reactor so thatoxygen is implanted with a vertical distribution mainly concentrated ina “box” 5216 (FIG. 120A) below a thin silicon surface layer 5217 (block5212 of FIG. 119). The goal is to leave the thin silicon surface layer5217 sufficiently free of defects (such as implanted oxygen atoms) topermit epitaxial growth of additional crystalline silicon on the surfacein a later step. The silicon surface layer 5217 is thin (e.g., about 3nm, for example) in order to accommodate the limited oxygen implantdepth of the typical plasma ion immersion implantation reactor. One wayof minimizing the implanted oxygen atom concentration in the thinsilicon surface layer 5217 is to concentrate the implanted oxygen atomsin the narrowest possible implanted depth range. For this purpose, theplasma is formed from water vapor, which dissociates into oxygen atomsand hydroxyl (OH) radicals. The advantage is that both species are ofnearly the same atomic mass and are therefore implanted at nearly thesame depth. This yields a narrow implanted oxygen depth profile, so thatthe oxygen distribution is spread less into the thin silicon surfacelayer 5217. In contrast, a plasma formed from oxygen gas dissociatesinto O2 ions and O ions which differ in atomic mass from one another bya factor of two so that the implanted depth distribution has two widelyseparate peaks, which would be less desirable.

The resulting implanted structure is illustrated in FIG. 120A, in whicha silicon substrate 5214 has an upper layer or “box” 5216 containingimplanted oxygen atoms. The implanted oxygen atoms have a peakconcentration well below the thin silicon surface layer 5217 (e.g., at adepth of 500 Angstroms), with a Gaussian distribution such that theconcentration of oxygen in the (e.g., 3 nm) silicon surface layerexceeds the threshold (e.g., about 10¹⁸ 8 cm−3) above which subsequentepitaxial silicon growth is prevented. The following steps are performedin order to reduce the concentration of oxygen in the silicon surfacelayer below that threshold.

In the next step (block 5218 of FIG. 119), an ion beam implant machineimplants atoms such as oxygen, silicon or hydrogen to form a deep damagelayer 5220 indicated by shading in FIG. 120B coinciding with the “box”5216 in which the implanted oxygen is distributed. The purpose offorming damaged layer below the surface is to increase oxygen diffusiontoward the peak or center of the oxygen implant distribution or profileduring the subsequent high temperature annealing step. The implantedatomic concentration in the damage layer 5220 is light (e.g., 10¹⁵ cm−3)so that the time to perform this implant step is relatively short. Thewafer is then heated or annealed (block 5221) at a very high temperature(1300 degrees C.) to promote migration or diffusion of the oxygen atomsin the implanted layer or “box” 5216 inwardly toward the center or peakof the oxygen implantation profile or distribution. The resultingnarrowing of the oxygen distribution (away from the silicon surfacelayer 5217) is indicated in FIG. 120C. The high temperature annealingstep may be carried out by dynamic surface annealing in which a laserbeam heats the silicon wafer surface to near the melting temperature ofsilicon for a very short time. Alternatively, the annealing step may becarried out by a rapid thermal annealing process in which wafer isheated by a radiant source such as a heat lamp. Or, it may be carriedout in a furnace in which the wafer is heated by convection. Theannealing step causes the implanted oxygen to migrate toward high oxygenconcentration region in 5216 and therefore away from the silicon surfacelayer 5217, thereby reducing the oxygen concentration in the siliconsurface layer 5217.

An oxygen-getter species, such as hydrogen, may be implanted in siliconsurface layer 5217 (block 5224 of FIG. 119), so that the surface layer5217 contains not only implanted silicon but also implanted hydrogen, asindicated in FIG. 120C. The hydrogen ion implantation may be carried outin the plasma immersion ion implantation reactor of FIG. 85.

In addition to implanting hydrogen in the silicon surface layer 5217,the substrate surface may also be treated with hydrogen (block 5230 ofFIG. 119) to clean or improve it for a subsequent epitaxial silicondeposition step. In the hydrogen surface treatment step of block 5230,some of the hydrogen interacts with oxygen atoms bonded to the siliconatoms near the substrate surface so as to remove such oxygen atoms. Insome cases, this may result in hydrogen atoms reacting with the surfacesilicon atoms and hence saturating some of the silicon dangling bonds,such that the surface becomes terminated with hydrogen atoms.

In the next step (block 5222 of FIG. 119), the silicon content of thesurface layer 5217 is enriched by implanting silicon into the siliconsurface layer 5217. The plasma immersion ion implantation reactor ofFIG. 85 may be employed in carrying out this step. The result isillustrated in FIG. 120C, in which the presence of the implanted siliconin the surface layer 5217 is indicated by shading.

An epitaxial silicon layer is grown on the top surface of the siliconsurface layer 5217 (block 5226 of FIG. 119). The resulting structure isillustrated in FIG. 120D, showing an epitaxial silicon layer 5228 overthe silicon surface layer 5217. The silicon surface layer 5217 at thisstage of the process contains implanted silicon and hydrogen. Thesilicon epitaxial growth step of block 5226 may be carried out in aconventional CVD plasma reactor.

An annealing step (block 5240 of FIG. 119) can be carried out tocomplete the epitaxial deposition process. The annealing step causes theimplanted silicon in the surface layer 5217 to be substituted into thecrystal lattice and replace any oxygen atoms that may have entered thecrystal lattice. The implanted silicon may also participate in aninternal epitaxial growth of a silicon crystal lattice. The resultingstructure is illustrated in FIG. 120E, in which the implanted oxygen“box” 5216 has shifted down and away from the silicon surface layer 5217toward the high oxygen concentration layers, the hydrogen atoms haveleft the crystal and the implanted silicon atoms have become a part ofthe crystal.

Finally, SOI transistors are formed in the epitaxial silicon layer(block 5500 of FIG. 119), the resulting transistor structures beingillustrated in FIG. 120F. The fabrication of the transistors of FIGS.120F may be carried out as described earlier in this specification withreference to the process of FIG. 112. In FIG. 120F, each transistor 5509is formed in the silicon layer 5217 and has a source 5519, a drain 5540and a gate 5560, on which are formed respective metal silicide contacts5521, 5541, 5561.

FIG. 121 illustrates a variation of the foregoing process in which thesilicon ion implantation step and the silicon epitaxial deposition stepare carried out at separate times in a silicon-containing plasma (ratherthan being performed simultaneously as in the modified step of block5022 of FIG. 16). The plasma immersion ion implantation reactor of FIG.85 may be employed for this purpose, in which the plasma bias voltageapplied to the wafer is first set to an intermediate level at whichsilicon ions accumulate on the substrate surface in an epitaxialdeposition process, and then the bias voltage is increased so thatsilicon ions are implanted in the silicon surface layer. The process ofFIG. 121 begins with heating of the wafer in the plasma immersion ionimplantation reactor (block 5710 of FIG. 121). Oxygen is implanted inthe manner described above with respect to the process of FIG. 119(block 5720 of FIG. 121. Then, a plasma containing silicon and,optionally, hydrogen, is formed in the P3i reactor (block 5740 of FIG.121). The precursor for such a plasma may be silane, for example. Thebias voltage is set to a sufficient level at which the silicon ions areimplanted in the silicon surface layer (i.e., within the first 3 nm ofthe surface) rather than simply accumulating on the surface, to performthe silicon implant step (block 5760 of FIG. 121). The bias voltageapplied to the wafer is then set to an intermediate value at whichsilicon ions tend to accumulate on the substrate surface rather thanbeing implanted. The chamber pressure and plasma density are selected inaccordance with well-known procedures for effecting epitaxial silicongrowth on the substrate surface (block 5750 of FIG. 121). The biasvoltage is, at some point, also set to a value at which hydrogen ionsfrom the plasma are implanted in the silicon surface layer (block 5765).The order of the silicon deposition and silicon implantation steps maybe reversed.

A deep damage layer is formed as in the process of FIG. 119 (block 5730of FIG. 121). Such a damage layer coincides with the original oxygendistribution. This damage helps the oxygen atoms to migrate from the lowoxygen concentration layers to the high oxygen concentration layers nearthe oxygen profile peak or center during the high temperature (>1300deg. C.) anneal step. The high oxygen concentration layers lie in themiddle of the implanted oxygen profile, while the low oxygenconcentration layers lie at the leading and falling edges of theimplanted oxygen profile. The next step is to anneal the wafer (block5770 of FIG. 121) as in the high temperature anneal step of the processof FIG. 119.

FIG. 122 illustrates another variation in which only the damage layerformation and high temperature annealing step are performed to reducethe oxygen content of the silicon surface layer, after which the step ofepitaxially depositing silicon is performed. Specifically, in FIG. 122,oxygen implantation and damage layer implantation steps 5810, 5820 arecarried out. A high temperature annealing step is performed (block5850). This has the effect of removing oxygen from the silicon surfacelayer by shifting the implanted oxygen down to the high oxygenconcentration layers by the Oswald ripening effect. At this point, thethin silicon surface layer is relatively free of oxygen (or at least itsoxygen content is less than 10¹⁸ cm−3). Then, the epitaxial silicondeposition step is carried out (block 5860).

In the process of FIG. 122, as well as in the preceding processes ofFIGS. 114, 116, 119 and 121, the formation of the damage layer may becarried out by plasma immersion ion implantation in the P3I reactor byapplying a very large wafer bias voltage (e.g., 40 kilovolts) using apulsed D.C. bias voltage source. Thus, the P3I reactor may be used forthe entire process of FIG. 122 if desired.

FIG. 123 illustrates modifications to the plasma immersion ionimplantation reactor of FIG. 85, in which either a radiant heat source(a lamp) 5910 is provided, with the wafer being thermally insulated fromthe wafer support pedestal, or the wafer pedestal itself is heated by aheater 5920, the wafer not being thermally insulated from the pedestal.In addition, a wafer temperature probe 5930 is provided in the wafersupport pedestal. Finally, a pulsed D.C. bias voltage source 5940 isconnected to the wafer pedestal, in lieu of an RF bias voltage source.The pulsed D.C. bias voltage source can apply a greater bias voltagebecause it is not susceptible to the type of reactive losses that plaguean RF source at high voltage. Moreover, the pulsed D.C. bias voltagesource can apply a greater bias voltage without creating a concomitantincrease in plasma ion density, so that plasma ion density is easier tocontrol at high bias voltage levels with a pulsed D.C. bias voltagesource. A pulsed D.C bias voltage of this type can also be used forimplanting the oxygen atoms in the step of block 5020 of FIGS. 114 and116, or block 5032 of FIG. 115, block 5212 of FIG. 119, or block 5720 ofFIG. 121 or block 5810 of FIG. 122.

FIG. 124 illustrates a complete system for carrying out the foregoingprocesses. The system includes a plasma immersion ion implantationreactor 5972 of the type illustrated in FIG. 123, an ion beamimplantation machine 5974, an anneal chamber 5976 and (optionally) achemical vapor deposition plasma reactor 5978. In addition, a robotwafer handler 5980 is coupled to each of the foregoing units 5972, 5974,5976, 5978 and a factory interface 5982 via sealed conduits. The plasmaimmersion ion implantation reactor 5972 performs the oxygen implant stepand the silicon implant step, the ion beam implantation machine 5974performs the deep damage layer implant step, the anneal chamber 5976performs the anneal step and the chemical vapor deposition reactor 5978performs the epitaxial silicon deposition step (if this step is notperformed in the plasma immersion ion implantation reactor 5972).

FIG. 125 illustrates a variation of the system of FIG. 124 in which theion beam implantation machine 5974 and the anneal chamber 5976 are notcoupled to the robot wafer handler 5980.

While the invention has been described in detail by specific referenceto preferred embodiments, it is understood that variations andmodifications thereof may be made without departing from the true spiritand scope of the invention.

1. A method of fabricating a silicon-on-insulator structure having asilicon layer in a semiconductor workpiece, said method comprising:placing said workpiece in a processing zone of a chamber bounded by achamber side wall and a chamber ceiling facing said workpiece andbetween a pair of ports of said chamber near generally opposite sides ofsaid processing zone and connected together by an external conduit ofsaid chamber; maintaining said workpiece at an elevated temperature;introducing into said chamber a process gas comprising oxygen;generating from said process gas a plasma current and causing saidplasma current to oscillate in a circulatory reentrant path comprisingsaid conduit and said processing zone.
 2. The method of claim 1 furthercomprising: applying a bias to said workpiece and setting said bias to alevel corresponding to an implant depth in said workpiece below saidsilicon layer to which oxygen atoms are to be implanted, whereby to forman oxygen-implanted layer in said workpiece having an oxygenconcentration distribution generally centered at said implant depth. 3.The method of claim 2 wherein the step of applying a bias to saidworkpiece comprises applying a pulsed D.C. bias voltage to saidworkpiece.
 4. The method of claim 2 wherein said oxygen concentrationdistribution has a finite oxygen concentration in said silicon layer,said method further comprising: reducing said oxygen concentration insaid silicon layer.
 5. The method of claim 4 wherein the step ofreducing said oxygen concentration in said silicon layer comprises:enriching the silicon content of said silicon layer by implantingsilicon atoms into said silicon layer and then heating said workpiece.6. The method of claim 4 wherein the step of reducing said oxygenconcentration in said silicon layer comprises: implanting anoxygen-getter species into said silicon layer and heating saidworkpiece.
 7. The method of claim 6 wherein said oxygen-getter speciescomprises hydrogen.
 8. The method of claim 4 wherein the step ofreducing said oxygen concentration in said silicon layer comprises: ionimplanting a damage layer at or near said implant depth of saidoxygen-implanted layer; heating said wafer so as to cause the implantedoxygen in at least said silicon layer to migrate away from said siliconlayer.
 9. The method of claim 8 wherein the step of ion implanting thedeep damage layer is carried out by ion beam implantation of an atomicspecies to an implanted concentration less than that of saidoxygen-implanted layer whereby to reduce the amount of time required toperform said ion beam implantation.
 10. The method of claim 9 whereinsaid atomic species comprises at least one of silicon, oxygen, hydrogen.11. The method of claim 9 wherein said implanted concentration of saiddeep damage layer is on the order of about 10¹⁵ cm−3.
 12. The method ofclaim 8 wherein the step of ion implanting the deep damage layer iscarried out by plasma immersion ion implantation.
 13. The method ofclaim 12 wherein the step of plasma immersion ion implantationcomprises: placing said workpiece in a processing zone of a chamberbounded by a chamber side wall and a chamber ceiling facing saidworkpiece and between a pair of ports of said chamber near generallyopposite sides of said processing zone and connected together by anexternal conduit of said chamber; maintaining said workpiece at anelevated temperature; introducing into said chamber a process gascomprising said atomic species; generating from said process gas aplasma current and causing said plasma current to oscillate in acirculatory reentrant path comprising said conduit and said processingzone; and applying a bias voltage on said workpiece corresponding to adeep implant depth below the implant depth of said oxygen-implantedlayer.
 14. The method of claim 4 wherein the step of reducing saidoxygen concentration in said silicon layer reduces said oxygenconcentration below a threshold value under which epitaxial growth of acrystalline silicon layer on said silicon layer can be carried out. 15.The method of claim 14 wherein said threshold value is about 10¹⁸ cm−3.16. The method of claim 14 further comprising: depositing an epitaxialsilicon layer on said silicon layer.
 17. The method of claim 4 whereinthe step of reducing said oxygen concentration in said silicon layercomprises: enriching the silicon content of said silicon layer byimplanting silicon atoms into said silicon layer; implanting anoxygen-getter species into said silicon layer; ion implanting a damagelayer at or near said implant depth of the implanted oxygen; heatingsaid wafer to a sufficient temperature and for a sufficient time tocause the implanted silicon to become substitutional in said siliconlayer, and implanted oxygen in or near said silicon layer to migrateaway from said silicon layer.
 18. The method of claim 17 wherein saidoxygen-getter species comprises hydrogen.
 19. The method of claim 17wherein the step of reducing said oxygen concentration in said siliconlayer reduces said oxygen concentration below a threshold value underwhich epitaxial growth of a crystalline silicon layer on said siliconlayer can be carried out.
 20. The method of claim 19 wherein saidthreshold value is about 10¹⁸ cm−3.
 21. The method of claim 4 whereinthe step of reducing said oxygen concentration in said silicon layercomprises: enriching the silicon content of said silicon layer byimplanting silicon atoms into said silicon layer; ion implanting a deepdamage layer generally at said implant depth of the implanted oxygen;heating said wafer so as to cause implanted oxygen in or near saidsilicon layer to migrate away from said silicon layer and the implantedsilicon to become substitutional in said silicon layer.
 22. The methodof claim 21 wherein the step of reducing said oxygen concentration insaid silicon layer reduces said oxygen concentration below a thresholdvalue under which epitaxial growth of a crystalline silicon layer onsaid silicon layer can be carried out.
 23. The method of claim 23wherein said threshold value is about 10¹⁸ cm−3.
 24. The method of claim1 wherein the step of generating a plasma current comprises coupling RFplasma source power into said conduit, whereby to cause said plasmacurrent to oscillate at a frequency of said RF plasma source power. 25.The method of claim 1 wherein said process gas comprising oxygencomprises water vapor.
 26. The method of claim 1 further comprising:immersing said workpiece in a plasma containing silicon ions whileapplying a bias voltage to said workpiece; setting said bias voltage ata level such that silicon atoms are implanted in said silicon layer;heating said workpiece.
 27. The method of claim 26 further comprisingdepositing an epitaxial layer of silicon on said silicon layer.
 28. Themethod of claim 27 wherein the step of depositing an epitaxial layer ofsilicon comprises setting said bias voltage to a level such that siliconatoms from said plasma accumulate on top of said silicon layer.
 29. Themethod of claim 27 wherein the step of depositing an epitaxial layer ofsilicon is performed contemporaneously with the step of implantingsilicon in said silicon layer by setting said bias voltage to a level atwhich some silicon atoms from said plasma accumulate on top of saidsilicon layer while other silicon atoms from said plasma are implantedin said silicon surface layer.
 30. The method of claim 26 wherein thestep of depositing an epitaxial layer of silicon is performed before thestep of heating the workpiece.
 31. The method of claim 26 wherein thestep of depositing an epitaxial layer of silicon is performed after thestep of heating the workpiece.
 32. The method of claim 26 wherein saidplasma further contains hydrogen ions, said method further comprising:setting said bias voltage to a level at which hydrogen atoms areimplanted in said silicon layer.
 33. The method of claim 26 wherein thestep of immersing said workpiece in a plasma containing silicon ionscomprises: placing said workpiece in a processing zone of a chamberbounded by a chamber side wall and a chamber ceiling facing saidworkpiece and between a pair of ports of said chamber near generallyopposite sides of said processing zone and connected together by anexternal conduit of said chamber; maintaining said workpiece at anelevated temperature; introducing into said chamber a process gascomprising silicon; generating from said process gas a plasma currentand causing said plasma current to oscillate in a circulatory reentrantpath comprising said conduit and said processing zone.
 34. A method offabricating a silicon-on-insulator structure having a silicon layer in asemiconductor workpiece, said method comprising: maintaining saidworkpiece at an elevated temperature; producing an oxygen-containingplasma in said chamber; applying a bias to said workpiece and settingsaid bias to a level corresponding to an implant depth in said workpiecebelow said silicon layer to which oxygen atoms are to be implanted,whereby to form an oxygen-implanted layer in said workpiece having anoxygen concentration distribution generally centered at said implantdepth and having a finite oxygen concentration in said silicon layer;reducing said oxygen concentration in said silicon layer.
 35. The methodof claim 34 wherein the step of applying a bias to said workpiececomprises applying a pulsed D.C. bias voltage to said workpiece.
 36. Themethod of claim 34 wherein the step of reducing said oxygenconcentration in said silicon layer comprises: enriching the siliconcontent of said silicon layer by implanting silicon atoms into saidsilicon layer and then heating said workpiece.
 37. The method of claim34 wherein the step of reducing said oxygen concentration in saidsilicon layer comprises: implanting an oxygen-getter species into saidsilicon layer and then heating said workpiece.
 38. The method of claim37 wherein said oxygen-getter species comprises hydrogen.
 39. The methodof claim 34 wherein the step of reducing said oxygen concentration insaid silicon layer comprises: ion implanting a damage layer at or nearsaid implant depth of the implanted oxygen; heating said wafer so as tocause the implanted oxygen in at least said silicon layer to migrateaway from said silicon layer.
 40. The method of claim 39 wherein thestep of ion implanting the deep damage layer is carried out by ion beamimplantation of one of oxygen, silicon, or hydrogen atoms to a reducedconcentration less than that of said oxygen-implanted layer.
 41. Themethod of claim 40 wherein said reduced concentration is on the order ofabout 10¹⁸ cm−3.
 42. The method of claim 34 wherein the step of reducingsaid oxygen concentration in said silicon layer reduces said oxygenconcentration below a threshold value below which epitaxial growth of acrystalline silicon layer on said silicon layer can be carried out. 43.The method of claim 42 wherein said threshold value is about 10¹⁸ cm−3.44. The method of claim 42 further comprising: depositing an epitaxialsilicon layer on said silicon layer.
 45. The method of claim 34 whereinthe step of reducing said oxygen concentration in said silicon layercomprises: enriching the silicon content of said silicon layer byimplanting silicon atoms into said silicon layer; implanting anoxygen-getter species into said silicon layer; ion implanting a damagelayer at or near said implant depth of the implanted oxygen; heatingsaid wafer so as to cause implanted oxygen in or near said silicon layerto migrate away from said silicon layer, and the implanted silicon tobecome substitutional in said silicon layer.
 46. The method of claim 45wherein said oxygen-getter species comprises hydrogen.
 47. The method ofclaim 45 wherein the step of reducing said oxygen concentration in saidsilicon layer reduces said oxygen concentration below a threshold valuebelow which epitaxial growth of a crystalline silicon layer on saidsilicon layer can be carried out.
 48. The method of claim 47 whereinsaid threshold value is about 10¹⁸ cm−3.
 49. The method of claim 34wherein the step of reducing said oxygen concentration in said siliconlayer comprises: enriching the silicon content of said silicon layer byimplanting silicon atoms into said silicon layer; ion implanting adamage layer at or near said implant depth of the implanted oxygen;heating said wafer so as to cause implanted oxygen in or near saidsilicon layer to migrate away from said silicon layer and the implantedsilicon to become substitutional in said silicon layer.
 50. The methodof claim 49 wherein the step of reducing said oxygen concentration insaid silicon layer reduces said oxygen concentration below a thresholdvalue below which epitaxial growth of a crystalline silicon layer onsaid silicon layer can be carried out.
 51. The method of claim 50wherein said threshold value is about 10¹⁸ cm−3.
 52. The method of claim34 further comprising: immersing said workpiece in a plasma containingsilicon ions while applying a bias voltage to said workpiece; settingsaid bias voltage at a level such that silicon atoms are implanted insaid silicon layer; heating said workpiece.
 53. The method of claim 52further comprising depositing an epitaxial layer of silicon on saidsilicon layer.
 54. The method of claim 53 wherein the step of depositingan epitaxial layer of silicon is performed before the step of heatingthe workpiece.
 55. The method of claim 53 wherein the step of depositingan epitaxial layer of silicon is performed after the step of heating theworkpiece.
 56. The method of claim 53 wherein the step of depositing anepitaxial layer of silicon comprises setting said bias voltage to alevel such that silicon atoms from said plasma accumulate on top of saidsilicon layer.
 57. The method of claim 53 wherein the step of depositingan epitaxial layer of silicon is carried out contemporaneously with thestep of implanting silicon into said silicon layer, wherein said biasvoltage is set to a level at which some silicon atoms from said plasmaaccumulate on top of said silicon layer while others of said siliconatoms are implanted in said silicon layer.
 58. The method of claim 52wherein the step of immersing said workpiece in a plasma containingsilicon and hydrogen comprises generating a plasma from a silane processgas in said reactor and causing said process gas to oscillate in saidcirculatory reentrant path.
 59. The method of claim 52 wherein saidplasma further contains hydrogen, said method further comprising settingsaid bias voltage to a level at which hydrogen atoms from said plasmaare implanted in said silicon layer.